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Don’t make your SOC blind to Active Directory attacks: 5 surprising behaviors of Windows audit…

6 July 2021 at 21:54

Don’t make your SOC blind to Active Directory attacks: 5 surprising behaviors of Windows audit policy

Tenable.ad can detect Active Directory attacks. To do this, the solution needs to collect security events from the monitored Domain Controllers to be analyzed and correlated. Fortunately, Windows offers built-in audit policy settings to configure which events should be logged. But when testing those options, we noticed surprising behaviors that can lead to missed events.

When you configure your Active Directory domain controllers to log security events to send to your SIEM and raise alerts, you absolutely do not want any regression which would ultimately blind your SOC! In this article we will share technical tips to prevent those unexpected issues.

Disclaimer

This content is based on observations and our interpretation of Microsoft documentation. This article is provided “as-is” and we do not provide any guarantee of correctness nor exhaustiveness and you should only rely on Microsoft guidance.

Introduction

Starting with Windows 2000, Windows offered only simple audit policy settings grouped in nine categories. Those are referred to as “top-level categories” or “basic audit policy” and they are still available in modern versions.

Later, “granular auditing” was introduced with Windows Vista / 2008 (it was configurable only via “auditpol.exe”) and then Windows 7 / 2008 R2 (configurable via GPO). Those are referred to as “sub-level categories” or “advanced audit policy”.

Each basic setting corresponds to a mix of several advanced settings. For example, from Microsoft Advanced security auditing FAQ:

Enabling the single basic account logon setting would be the equivalent of setting all four advanced account logon settings.

The content described in this article was tested on Windows Active Directory domain controllers because those are the most appropriate sources of interest for Active Directory attacks detection, but it should apply to all kinds of Windows machines (servers & workstations).

Surprise #1 — Advanced audit policy fully replaces the basic policy

As soon as we enable even just one advanced audit policy setting, Windows fully switches to advanced policy mode and ignores all existing basic policies (at least on the recent versions of Windows we tested)! Here is a demonstration:

  • Before: the system uses basic settings. We enable “Success, Failure” for “Audit privilege use” (green highlighting) and for other categories the default values apply. This works as expected:
  • After: we only enable one advanced setting (green highlighting). Notice how everything else is not audited anymore, including what we explicitly configured in the basic policy (red highlighting)!

Therefore, you cannot have both and thus when you start using the advanced audit policy, which you should, you are committed to it and should abandon the basic settings to prevent confusion.

Microsoft Advanced security auditing FAQ explains it:

When advanced audit policy settings are applied by using Group Policy, the current computer’s audit policy settings are cleared before the resulting advanced audit policy settings are applied. After you apply advanced audit policy settings by using Group Policy, you can only reliably set system audit policy for the computer by using the advanced audit policy settings. […] Important: Whether you apply advanced audit policies by using Group Policy or by using logon scripts, do not use both the basic audit policy settings under Local Policies\Audit Policy and the advanced settings under Security Settings\Advanced Audit Policy Configuration. Using both advanced and basic audit policy settings can cause unexpected results in audit reporting.

➡️ Tenable.ad recommendation: use advanced audit policy settings only. Existing basic audit policies should be converted.
This recommendation is present in the best practices and hardening guides published by cybersecurity organizations (such as ANSSI, DISA STIG, CIS Benchmarks…).

Surprise #2 — Advanced audit policy may be ignored

However, there are some cases where basic audit policy settings may still take priority over the ones defined in the advanced audit policy. Correctly understanding when and where it could happen is complicated.

As per Microsoft Advanced security auditing FAQ:

If you use Advanced Audit Policy Configuration settings or use logon scripts to apply advanced audit policies, be sure to enable the “Audit: Force audit policy subcategory settings (Windows Vista or later) to override audit policy category settings” policy setting under “Local Policies\Security Options”. This will prevent conflicts between similar settings by forcing basic security auditing to be ignored.

➡️ Tenable.ad recommendation: once you start using advanced audit policy, we recommend enabling the “Audit: Force audit policy subcategory settings (Windows Vista or later) to override audit policy category settings” GPO setting to prevent undesired surprises. Its default value being “Enabled”, it should already be effective anyway in the majority of environments.
This recommendation is present in the best practices and hardening guides published by cybersecurity organizations (such as ANSSI, DISA STIG, CIS Benchmarks…).

Surprise #3 — Advanced audit policy default values are not respected

As we saw previously, as soon as we enable even just one advanced audit policy setting the system entirely switches to the advanced mode. The question we may have now is how does the system manage the other settings that we did not specify? There are certainly sensible default values, aren’t there? These default values are described in the documentation of each audit policy setting. Let’s read the explanation of the “Audit Logon” setting:

So, here on a server I should expect a default value of “Success, Failure” for the “Audit Logon” setting if not configured, shouldn’t I? Well, we may have a surprise here.

Here is the configuration I applied on my server: I enabled “Success” logging for “Audit Account Lockout” and left “Audit Logon” as “Not Configured”:

However, when looking at the resulting audit policy I notice that “Logon” events are not audited, contrary to their default:

We knew we should not rely on defaults… but this one is really surprising. Of course we made sure that there was no other GPO defining any audit policy setting.

➡️ Tenable.ad recommendation: do not rely on default values for Advanced audit policy settings: explicitly configure the desired value (No Auditing, Success, Failure, or Success and Failure) for each setting of interest.

Be even more careful when migrating from a basic audit policy: make sure to export the resulting policy you had on a normal machine, and convert it to all the appropriate advanced settings to prevent any regression in logging. And as usual with GPOs, especially for security settings, aim to create a single security GPO linked the highest possible, instead of spreading those in many lower-level GPOs.

Surprise #4 — Settings defined by GPOs are not merged

What happens when a machine is covered by several GPOs which define audit policy settings? What if one GPO enables “Success” auditing while another enables “Failure” auditing, is there a merge and would we obtain “Success and Failure”?

Answer: there is no merge at the setting level, and only the value of the GPO with the highest priority is applied. This is actually coherent with the way the Group Policy engine usually works, so not really a surprise, but still to keep in mind.

Here is a demonstration where we want to configure auditing on domain controllers. Two GPOs apply to those servers:

Default Domain Policy” linked at the top of the Active Directory domain

  • Audit Account Lockout” is set to “Success and Failure” (yellow highlighting)
  • Audit Logon” is set to “Success” (red highlighting)

Default Domain Controllers Policy” linked to the “Domain Controllers” organization unit

  • Audit Logoff” is set to “Success and Failure” (blue highlighting)
  • Audit Logon” is set to “Failure” (red highlighting)

Now let’s see the resulting audit policy:

We notice that the conflicting values for “Logon” (red highlighting) were not merged, instead it is the value of the “Default Domain Controllers Policy”. This GPO won as per the usual GPO precedence rules.

We also observe that the values for “Logoff” (blue highlighting) from the “Default Domain Controllers Policy” and “Account Lockout” (yellow highlighting) from the “Default Domain Policy” are both properly applied because those were not in conflict.

Here is how Microsoft Advanced security auditing FAQ explains it:

By default, policy options that are set in GPOs and linked to higher levels of Active Directory sites, domains, and OUs are inherited by all OUs at lower levels. However, an inherited policy can be overridden by a GPO that is linked at a lower level.

You can also read more about GPO Processing Order in the [MS-GPOL] specification.

➡️ Tenable.ad recommendation: keep in mind that conflicting audit settings are not merged.
If you want to define a domain-wide security auditing GPO, you should ensure that no other GPO at a lower OU level overrides its settings. If necessary, you can set this domain-wide GPO as “Enforced”, even if this is not our preferred option as it can become confusing when managing a large set of GPOs.

If you are only concerned about auditing on domain controllers, you can link a GPO to the “Domain Controllers” organizational unit, as long as there is no domain-level “Enforced” GPO overriding audit policy settings.

Surprise #5 — Only one tool properly shows the effective audit policy

We have just shown that we can have many surprises when configuring auditing, so we really would like a way to see the effective audit policy on a system to confirm that it is as expected.

We could be tempted to use tools which compute the result of GPOs (RSoP), but…

For example, “rsop.msc” does not even seem to support advanced audit policy, which is not too surprising since it is deprecated! See how this section is used in the GPO editor on the right-hand side whereas it is missing in “rsop.msc” on the left-hand side:

And with “gpresult.exe”, if we have basic and advanced audit policies, we will see both: which one applies?

And what about settings that might have been configured locally and not through a GPO (which is not advised…)?

The only supported tool which can properly read the current effective audit policy is “auditpol.exe”, as you may have guessed from our previous screenshots. This is confirmed by a Microsoft blog post. For those who want to dig deeper: “auditpol.exe” calls AuditQuerySystemPolicywhich finally calls the “LsarQueryAuditPolicy” RPC in LSASS.

➡️ Tenable.ad recommendation: only trust the following command to see the effective audit policy on machines: “auditpol.exe /get /category:*”

Surprise bonus — Confusions in the specification

Configuring advanced audit policy in a GPO creates an “audit.csv” file which is described in the [MS-GPAC] Microsoft open specification. We found a mistake in one of the examples:

Machine Name,Policy Target,Subcategory,Subcategory GUID,Inclusion Setting,Exclusion Setting,Setting Value
TEST-MACHINE,System,IPsec Driver,{0CCE9213–69AE-11D9-BED3–505054503030},No Auditing,,0
TEST-MACHINE,System,System Integrity,{0CCE9212–69AE-11D9-BED3–505054503030},Success,,1
TEST-MACHINE,System,IPsec Extended Mode,{0CCE921A-69AE-11D9-BED3–505054503030},Success and Failure,,3
TEST-MACHINE,System,File System,{0CCE921D-69AE-11D9-BED3–505054503030},Not specified,,0

On the right-hand columns we have the setting name (such as “No Auditing”, “Success”, etc.) and the corresponding numerical value (0, 1, 3…). We can see that according to the first and last lines the value “0” is associated with both “No Auditing” and “Not specified” which does not make sense. Fortunately the text value is ignored: “value of InclusionSetting is for user readability only and is ignored when the advanced audit policy is applied”.

Also, we found the specification a bit confusing regarding the values of “0” and “4”:

A value of “0”: Indicates that this audit subcategory setting is unchanged.
A value of “4”: Indicates that this audit subcategory setting is set to None.

Our observations actually show that:

  • A value of “0” means that auditing is “disabled”, which corresponds to this in the graphical editor:
  • A value of “4” means that auditing is “not specified”, and thus the default value should apply (except when it does not, as shown before), which would correspond to this in the graphical editor (except that in this case the editor does not even generate a line for this setting in “audit.csv”):

Don’t make your SOC blind to Active Directory attacks: 5 surprising behaviors of Windows audit… was originally published in Tenable TechBlog on Medium, where people are continuing the conversation by highlighting and responding to this story.

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