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Bypassing Authentication on Arcadyan Routers with CVE-2021–20090 and rooting some Buffalo

3 August 2021 at 13:03

A while back I was browsing Amazon Japan for their best selling networking equipment/routers (as one does). I had never taken apart or hunted for vulnerabilities in a router and was interested in taking a crack at it. I came across the Buffalo WSR-2533DHP3 which was, at the time, the third best selling device on the list. Unfortunately, the sellers didn’t ship to Canada, so I instead bought the closely related Buffalo WSR-2533DHPL2 (though I eventually got my hands on the WSR-2533DHP3 as well).

In the following sections we will look at how I took the Buffalo devices apart, did a not-so-great solder job, and used a shell offered up on UART to help find a couple of bugs that could let users bypass authentication to the web interface and enable a root BusyBox shell on telnet.

At the end, we will also take a quick look at how I discovered that the authentication bypass vulnerability was not limited to the Buffalo routers, and how it affects at least a dozen other models from multiple vendors spanning a period of over ten years.

Root shells on UART

It is fairly common for devices like these Buffalo routers to offer up a shell via a serial connection known as Universal Asynchronous Receiver/Transmitter (UART) on the circuit board. Manufacturers often leave test points or unpopulated pads on the circuit board for accessing UART. These are often used for debugging or testing the device during manufacture. In this case, we were extremely lucky that, after some poor soldering and testing, the WSR-2533DHPL2 offered up a BusyBox shell as root over UART.

In case this is new to anyone, let’s quickly walk through this process (there are many articles out there on the web with a more detailed walkthrough on hardware hacking and UART shells).

The first step is for us to open up the router’s case and try to identify if there is a way to access UART.

UART interface on the WSR-2533DHP3

We can see a header labeled J4 which may be what we’re looking for. The next step is to test the contacts with a multimeter to identify power (VCC), ground (GND), and our potential transmit/receive (TX/RX) pins. Once we’ve identified those, we can solder on some pins and connect them to a tool like JTAGulator to identify which pins we will communicate on, and at what baud rate.

Don’t worry, this isn’t my usual setup, just a shameless plug

We could identify this in other ways, but the JTAGulator makes it much easier. After setting the voltage we’re using (3.3V found using the multimeter earlier) we can run a UART scan which will try sending a carriage-return (or some other specified bytes) and receiving on each pin, at different bauds, which helps us identify what combination thereof will let us communicate with the device.

Running a UART scan on JTAGulator

The UART scan shows that sending a carriage return over pin 0 as TX, with pin 2 as RX, and a baud of 57600, gives an output of BusyBox v1, which looks like we may have our shell.

UART scan finding the settings we need

Sure enough, after setting the JTAGulator to UART Passthrough mode (which allows us to communicate with the UART port) using the settings we found with the UART scan, we are dropped into a root shell on the device.

We can now use this shell to explore the device, and transfer any interesting binaries to another machine for analysis. In this case, we grabbed the httpd binary which was serving the device’s web interface.

Httpd and web interface authentication

Having access to the httpd binary makes hunting for vulnerabilities in the web interface much easier, as we can throw it into Ghidra and identify any interesting pieces of code. One of the first things I tend to look at when analyzing any web application or interface is how it handles authentication.

While examining the web interface I noticed that, even after logging in, no session cookies are set, and no tokens are stored in local/session storage, so how was it tracking who was authenticated? Opening httpd up in Ghidra, we find a function named evaluate_access() which leads us to the following snippet:

Snippet from FUN_0041fdd4(), called by evaluate_access()

FUN_0041f9d0() in the screenshot above checks to see if the IP of the host making the current request matches that of an IP from a previous valid login.

Now that we know what evaluate_access() does, lets see if we can get around it. Searching for where it is referenced in Ghidra we can see that it is only called from another function process_request() which handles any incoming HTTP requests.

process_request() deciding if it should allow the user access to a page

Something which immediately stands out is the logical OR in the larger if statement (lines 45–48 in the screenshot above) and the fact that it checks the value of uVar1 (set on line 43) before checking the output of evaluate_access(). This means that if the output of bypass_check(__dest) (where __dest is the url being requested) returns anything other than 0, we will effectively skip the need to be authenticated, and the request will go through to process_get() or process_post().

Let’s take a look at bypass_check().

Bypassing checks with bypass_check()

the bypass_list checked in bypass_check()

Taking a look at bypass_check() in the screenshot above, we can see that it is looping through bypass_list, and comparing the first n bytes of _dest to a string from bypass_list, where n is the length of the string grabbed from bypass_list. If no match is found, we return 0 and will be required to pass the checks in evaluate_access(). However, if the strings match, then we don’t care about the result of evaluate_access(), and the server will process our request as expected.

Glancing at the bypass list we see login.html, loginerror.html and some other paths/pages, which makes sense as even unauthenticated users will need to be able to access those urls.

You may have already noticed the bug here. bypass_check() is only checking as many bytes as are in the bypass_list strings. This means that if a user is trying to reach http://router/images/someimage.png, the comparison will match since /images/ is in the bypass list, and the url we are trying to reach begins with /images/. The bypass_check() function doesn’t care about strings which come after, such as “someimage.png”. So what if we try to reach /images/../<somepagehere>? For example, let’s try /images/..%2finfo.html. The /info.html url normally contains all of the nice LAN/WAN info when we first login to the device, but returns any unauthenticated users to the login screen. With our special url, we might be able to bypass the authentication requirement.

After a bit of match/replace to account for relative paths, we still see an underwhelming display. We have successfully bypassed authentication using the path traversal (🙂 ) but we’re still missing something (🙁 ).

404s for requests to made to js files

Looking at the Burp traffic, we can see a number of requests to /cgi/<various_nifty_cgi>.js are returning a 404, which normally return all of the info we’re looking for. We also see that there are a couple of parameters passed when making requests to those files.

One of those parameters (_t) is just a datetime stamp. The other is an httoken, which acts like a CSRF token, and figuring out where / how those are generated will be discussed in the next section. For now, let’s focus on why these particular requests are failing.

Looking at httpd in Ghidra shows that there is a fair amount of debugging output printed when errors occur. Stopping the default httpd process, and running it from our shell shows that we can easily see this output which may help us identify the issue with the current request.

requests failing due to improper Referrer header

Without diving into url_token_pass, we can see that it is saying that httoken is invalid from http://192.168.11.1/images/..%2finfo.html. We will dive into httokens next, but the token we have here is correct, which means that the part causing the failure is the “from” url, which corresponds to the Referer header in the request. So, if we create a quick match/replace rule in Burp Suite to fix the Referer header to remove the /images/..%2f then we can see the info table, confirming our ability to bypass authentication.

our content loaded :)

A quick summary of where we are so far:

  • We can bypass authentication and access pages which should be restricted to authenticated users.
  • Those pages include access to httokens which let us make GET/POST requests for more sensitive info and grant the ability to make configuration changes.
  • We know we also need to set the Referer header appropriately in order for httokens to be accepted.

The adventure of getting proper httokens

While we know that the httokens are grabbed at some point on the pages we access, we don’t know where they’re coming from or how they’re generated. This will be important to understand if we want to carry this exploitation further, since they are required to do or access anything sensitive on the device. Tracking down how the web interface produces these tokens felt like something out of a Capture-the-Flag event.

The info.html page we accessed with the path traversal was populating its information table with data from .js files under the /cgi/ directory, and was passing two parameters. One, a date time stamp (_t), and the other, the httoken we’re trying to figure out.

We can see that the links used to grab the info from /cgi/ are generated using the URLToken() function, which sets the httoken (the parameter _tn in this case) using the function get_token(), but get_token() doesn’t seem to be defined anywhere in any of the scripts used on the page.

Looking right above where URLToken() is defined we see this strange string defined.

Looking into where it is used, we find the following snippet.

Which, when run adds the following script to the page:

We’ve found our missing getToken() function, but it looks to be doing something equally strange as the snippets that got us here. It is grabbing another encoded string from an image tag which appears to exist on every page (with differing encoded strings). What is going on here?

getToken() is getting data from this spacer img tag

The httokens are being grabbed from these spacer img src strings and are used to make requests to sensitive resources.

We can find a function where the httoken is being inserted into the img tag in Ghidra.

Without going into all of the details around the setting/getting of httoken and how it is checked for GET and POST requests, we will say that:

  • httokens, which are required to make GET and POST requests to various parts of the web interface, are generated server-side.
  • They are stored encoded in the img tags at the bottom of any given page when it loads
  • They are then decoded in client-side javascript.

We can use the tokens for any requests we need as long as the token and the Referer being used in the request match. We can make requests to sensitive pages using the token grabbed from login.html, but we still need the authentication bypass to access some actions (like making configuration changes).

Notably, on the WSR-2533DHPL2 just using this knowledge of the tokens means we can access the administrator password for the device, a vulnerability which appears to already be fixed on the WSR-2533DHP3 (despite both having firmware releases around the same time).

Now that we know we can effectively perform any action on the device without being authenticated, let’s see what we can do with that.

Injecting configuration options and enabling telnetd

One of the first places I check for any web interface / application which has utilities like a ping function is to see how those utilities are implemented, because even just a quick Google turns up a number of historic examples of router ping utilities being prone to command injection vulnerabilities.

While there wasn’t an easily achievable command injection in the ping command, looking at how it is implemented led to another vulnerability. When the ping command is run from the web interface, it takes an input of the host to ping.

After the request is made successfully, ARC_ping_ipaddress is stored in the global configuration file. Noting this, the first thing I tried was to inject a newline/carriage return character (%0A when url-encoded), followed by some text to see if we could inject configuration settings. Sure enough, when checking the configuration file, the text entered after %0A appears on a new line in the configuration file.

With this in mind, we can take a look at any interesting configuration settings we see, and hope that we’re able to overwrite them by injecting the ARC_ping_ipaddress parameter. There are a number of options seen in the configuration file, but one which caught my attention was ARC_SYS_TelnetdEnable=0. Enabling telnetd seemed like a good candidate for gaining a remote shell on the device.

It was unclear whether simply injecting the configuration file with ARC_SYS_TelnetdEnable=1 would work, as it would then be followed by a conflicting setting later in the file (as ARC_SYS_TelnetdEnable=0 appears lower in the configuration file than ARC_ping_ipdaddress). However, after sending the following request in Burp Suite, and sending a reboot request (which is necessary for certain configuration changes to take effect).

Once the reboot completes we can connect to the device on port 23 where telnetd is listening, and are greeted with a root BusyBox shell, just like we have via UART.

Altogether now

Here are the pieces we need to put together in a python script if we want to make exploiting this super easy:

  • Get proper httokens from the img tags on a page.
  • Use those httokens in combination with the path traversal to make a valid request to apply_abstract.cgi
  • In that valid request to apply_abstract.cgi, inject the ARC_SYS_TelnetdEnable=1 configuration option
  • Send another valid request to reboot the device
Running a quick PoC against the WSR-2533DHPL2

Surprise: More affected devices

Shortly before the 90 day disclosure date for the vulnerabilities discussed in this blog, I was trying to determine the number of potentially affected devices visible online via Shodan and BinaryEdge. In my searches, I noticed that a number of devices which presented similar web interfaces to those seen on the Buffalo devices. Too similar, in fact, as they appeared to use almost all the same strange methods for hiding the httokens in img tags, and javascript functions obfuscated in “enkripsi” strings.

The common denominator is that all of the devices were manufactured by Arcadyan. In hindsight, it should have been obvious to look for more affected devices outside of Buffalo’s product line given how much of the Buffalo firmware appeared to have been built by Arcadyan. However, after obtaining and testing a number of Arcadyan-manufactured devices it also became clear that not all of them were created equally, and the devices weren’t always affected in exactly the same way.

That said, all of the devices we were able to test or have tested via third-parties shared at least one vulnerability: The path traversal which allows an attacker to bypass authentication, now assigned as CVE-2021–20090. This appears to be shared by almost every Arcadyan-manufactured router/modem we could find, including devices which were originally sold as far back as 2008.

On April 21st, 2021, Tenable reported CVE-2021–20090 to four additional vendors (Hughesnet, O2, Verizon, Vodafone), and reported the issues to Arcadyan on April 22nd. As time went on it became clear that many more vendors were affected and contacting and tracking them all would become very difficult, and so on May 18th, Tenable reported the issues to the CERT Coordination Center for help with that process. A list of the affected devices can be found in either Tenable’s own advisory, and more information can be found on CERT’s page tracking the issue.

There is a much larger conversation to be had about how this vulnerability in Arcadyan’s firmware has existed for at least 10 years and has therefore found its way through the supply chain into at least 20 models across 17 different vendors, and that is touched on in a whitepaper Tenable has released.

Takeaways

The Buffalo WSR-2533DHPL2 was the first router I’d ever purchased for the purpose of discovering vulnerabilities, and it was a super fun experience. The strange obfuscations and simplicity of the bugs made it feel like my own personal CTF. While I got a little more than I bargained for upon learning how widespread one of the vulnerabilities (CVE-2021–20090) was, it was an important lesson in how one should approach research on consumer electronics: The vendor selling you the device is not necessarily the one who manufactured it, and if you find bugs in a consumer router’s firmware, they could potentially affect many more vendors and devices than just the one you are researching.

I’d also like to encourage security researchers who are able to get their hands on one of the 20+ affected devices to take a look for (and report) any post-authentication vulnerabilities like the configuration injection found in the Buffalo routers. I suspect there are a lot more issues to be found in this set of devices, but each device is slightly different and difficult to obtain for researchers not living in the country where they are sold/provided by a local ISP.

Thanks for reading, and happy hacking!


Bypassing Authentication on Arcadyan Routers with CVE-2021–20090 and rooting some Buffalo was originally published in Tenable TechBlog on Medium, where people are continuing the conversation by highlighting and responding to this story.

Stealing tokens, emails, files and more in Microsoft Teams through malicious tabs

14 June 2021 at 13:03

Trading up a small bug for a big impact

Intro

I recently came across an interesting bug in the Microsoft Power Apps service which, despite its simplicity, can be leveraged by an attacker to gain persistent read/write access to a victim user’s email, Teams chats, OneDrive, Sharepoint and a variety of other services by way of a malicious Microsoft Teams tab and Power Automate flows. The bug has since been fixed by Microsoft, but in this blog we’re going to see how it could have been exploited.

In the following sections, we’ll take a look at how we, as baduser(at)fakecorp.ca, a member of the fakecorp.ca organization, can create a malicious Teams tab and use it to eventually steal emails, Teams messages, and files from gooduser(at)fakecorp.ca, and send emails and messages on their behalf. While the attack we will look at has a lot of moving parts, it is fairly serious, as the compromise of business email is said to have cost victims $1.8 billion in 2020.

As an example to get us started, here is a quick clip of this method being used by Bad User to steal a Word document from Good User’s private OneDrive for Business.

Teams Tabs, Power Apps and Power Automate Flows

If you are already familiar with Teams and the Power Platform, feel free to skip this section, but otherwise, it may be useful to go over the pieces of the puzzle we’ll be using later.

Microsoft Teams has a default feature that allows a user to launch small applications as a tab in any team they are part of. If that user is part of an Office 365/Teams organization with a Business Basic license or above, they also have access to a set of Teams tabs which consist of Microsoft Power Apps applications.

A Teams tab with the Bulletins Power App

Power Apps are part of the wider Microsoft Power Platform, and when a user of a particular team launches their first Power App tab, it creates what Microsoft calls a “Dataverse for Teams Environment”, which according to Microsoft “is used to store, manage, and share team-specific data, apps, and flows”.

It should also be noted that, apart from the team-specific environments, there is a default environment for the organization as a whole. This is important because users can only create connectors and flows in either the default environment, or for teams which they own, and the attack we’re going to look at requires the ability to create Power Automate flows.

Power Automate is a service which lets users create automated workflows which can operate on their Office 365 organization’s data. For example, these flows can be used to do things like send emails on a particular schedule, or send Microsoft Teams messages any time a file on Sharepoint is updated.

Power Automate flow templates

The bug: trusting a bad domain

When a Power App tab is first created for a team, it runs through a deployment process that uses information gathered from the make.powerapps.com domain to install the application to the team dataverse/environment.

Installing the app

Teams tabs generally operate by opening an iframe to a page on a domain which is specified as trusted in that application’s manifest. What we see in the above image is a tab that contains an iframe to the page apps.powerapps.com/teams/makerportal?makerPortalUrl=https://make.powerapps.com/somePageHere, which itself is opening an iframe to the make.powerapps.com page passed in makerPortalUrl.

Immediately upon seeing this I was curious if I could make the apps.powerapps.com page load our own content. I noticed a couple of things:

  1. The apps.powerapps.com page will only load the iframe to makerPortalUrl if it is in a Microsoft Teams tab (it uses the Microsoft Teams javascript client sdk).
  2. The child iframe would only load if the makerPortalUrl begins with https://make.powerapps.com

We can see this happen if we view the page’s source, testing out different parameters. Trying to load any url which doesn’t begin with https://make.powerapps.com results in the makerPortalUrl being set to an empty string. However, the validation stops at checking whether the domain begins with make.powerapps.com, and does not check whether it is the full domain.

So, if we set makerPortalUrl equal to something like https://make.powerapps.com.fakecorp.ca/ we will be able to load our own content in the iframe!

Cool, we can load an iframe with our own content two iframes deep in a Teams tab, but what does that get us? Microsoft Teams already has a website tab type which lets you load an iframe with a URL of your choosing, and with those you can’t do much. Fortunately for us, some tabs have more capabilities than others.

Stealing auth tokens with postMessage

We can load our own content in an iframe, which itself is sitting in an iframe on apps.powerapps.com. The reason this is more interesting than something like the Website tab type on Teams is that for Power App extension tab types, the app.powerapps.com page communicates both with Teams, by way of the Teams JS SDK, as well as its child iframe using javascript postMessage.

We can communicate with the parent window via postMessage

Using a Chrome extension, we can watch the postMessages passed between windows as an application is installed and launched. At first glance, the most interesting message is a postMessage from make.powerapps.com in the innermost window (the window which we are replacing when specifying our own makerPortalUrl) to the apps.powerapps.com window, with GET_ACCESS_TOKEN in the data.

The frame which we were replacing was getting access tokens from its parent window without passing any sort of authentication.

the child iframe requesting an access token via postMessage

I tested this same kind of postMessage from the make.powerapps.com.fakecorp.ca subdomain, and sure enough, I was able to grab the same access tokens. A handler is registered in the WebPlayer.EmbedMakerPortal.js file loaded by apps.powerapps.com which fetches tokens for the requested resource using the https://apps.powerapps.com/auth/onbehalfof endpoint, which in our testing is capable of grabbing tokens for:

- apihub.azure.com
- graph.microsoft.com
- dynamics apps subdomains
- service.flow.microsoft.com
- service.powerapps.com
Grabbing the access token from a page we control

This is a super exciting thing to see: A tab under our control which can be created in a public team can retrieve access tokens on behalf of the user viewing it. Let’s slow down for a moment though, because I forgot to show an important step: how did we get our own content in a tab in the first place?

Overwriting a Teams tab

I mentioned earlier that Teams tabs generally operate by opening an iframe to a page which is specified in the tab application’s manifest. The request to define what page is loaded by a tab can be seen when adding a new tab or even renaming a currently existing tab.

The PUT request for renaming a tab lets us change the tab url

The url being given in this PUT request is pointing to the Bulletins Power App which is installed in our team environment. To point the tab to our malicious content we simply have to replace that url with our apps.powerapps.com/teams/makerportal?makerPortalUrl=https://make.powerapps.com.fakecorp.ca page.

It should be noted that this only works because we are passing a url with a trusted domain (apps.powerapps.com) according to the application’s manifest. If we try to pass malicious content directly as the tab’s url, the tab will not load our content.

A short and inconspicuous proof of concept

While the attacks we will look at later are longer and overly noisy for demonstration purposes, let’s consider a very quick proof of concept of how we could use what we currently have to steal access tokens from unsuspecting users.

If we host a page similar to the following and overwrite a tab to point to it, we can grab users’ service.flow.microsoft.com token and send it to another listener we control, while also loading the original Power App in an iframe that matches the tab size. While it won’t look exactly like a normally-running Power App tab, it doesn’t look different enough to notice. If the application requires postMessage communication with the parent app, we could even act as a man-in-the-middle for the postMessages being sent and received by adding a message handler to the PoC.

During the loading you can see two spinning circles. The smaller one is our JS running.

Now that we know we can steal certain tokens, let’s see what we can do with them, specifically the service.flow.microsoft.com token we just stole.

Stealing more tokens, emails, messages and files

The reason we’re focused on the service.flow.microsoft.com token is because it can be used to get us access to more tokens, and to create Power Automate flows, which will allow us to access a user’s email from Outlook, Teams messages, files from OneDrive and SharePoint, and a whole lot more.

We will construct the attack, at a high level, by:

- Grabbing an extra set of tokens from api.flow.microsoft.com
- Creating connectors to the services we want to access.
- Consent on behalf of the victim user using first party logins
- Creating Power Automate flows on the victim user’s behalf which let us send/receive emails and teams messages, retrieve emails, messages and files.
- Adding ourselves (or a group we’re in) to the owners of the flow.
- Having the victim user send an email to us containing any information we need to access the flows.

For our example we’re going to be showing pieces of a proof of concept which creates:

- Office 365 (for outlook access), and Teams connectors
- A flow which lets us send emails as the user
- A flow which lets us get all Teams messages from channels the victim is in, and send messages on their behalf.

The api.flow.microsoft.com token bundle

The first stop on our quest to get access to everything the victim user holds dear is an api endpoint which will let us generate a handful of new access tokens. Sending an empty POST request to api.flow.microsoft.com/providers/Microsoft.ProcessSimple/environments/<environment>/users/me/onBehalfOfTokenBundle?api-version=2021–01–03 will let us grab the following tokens, with the following scopes:

the api.flow.microsoft.com token bundle
- graph.microsoft.com
- scope : Contacts.Read Contacts.Read.Shared Group.Read.All TeamsAppInstallation.ReadWriteForTeam TeamsAppInstallation.ReadWriteSelfForChat User.Read User.ReadBasic.All
- graph.microsoft.net
- scope : user_impersonation
- appservice.azure.com
- scope : user_impersonation
- apihub.azure.com
- scope : user_impersonation
- consent.msp.windows.net/logic-app-aad
- scope : user_impersonation
- service.powerapps.com
- scope : user_impersonation

Some of these tokens will become useful to us for constructing a larger attack (specifically the graph.microsoft.com and apihub.azure.com tokens).

Creating connectors and using first party logins

To create flows which let us take control of the victim’s services, we first need to create connectors for those services.

When a connector is created, a user can use a consent link to login via a login.microsoft.com popup and grant permissions for the service for which the connector is being made (like Office 365, Teams, or Sharepoint). Some connectors, however, come with a first party login url, which lets us bypass the regular interactive login process and authorize the connector using only the authorization tokens already gathered.

Creating a connector on the victim’s behalf takes only three requests, the final of which is a POST request to the first party login url, with the apihub.azure.com access token.

consenting to a connector with a stolen apihub.azure.com token

After this third request, the connector will be ready to use with any flow we create.

Creating a flow

Given the number of potential connector types, flow triggers, and actions we can perform, there are an endless number of ways that we could leverage this access. They range anywhere from simply forwarding every email which is received by the victim to the attacker, to only performing actions if a particular RSS feed updates, to creating REST endpoints that let us trigger any number of different actions in different services.

Additionally, if the organization happens to have premium Power Apps/Automate licensing, there are many more options available. It is honestly a very useful system (even if you’re not trying to exploit a whole Office 365 org).

For our attack, we will look at creating a flow which gives us access to endpoints which take JSON input, and perform the actions we want (send emails, teams messages, downloads files, etc). It is a noisier method, since it requires the attacker to send requests (authenticated as themselves), but it is convenient for demonstration. Not all flows require the attacker to be authenticated, or require user interaction.

Choosing flow triggers

A flow trigger is how a flow will be kicked off / knows when to begin. The three main types are automatic (when an email comes in, forward it to this address), instant (when a request is received at this endpoint, trigger the flow), and scheduled (run the flow every xyz seconds/minutes/hours).

The flow trigger we would prefer to use is the “when an HTTP request is received” trigger, which lets unauthenticated users trigger the flow, but that is a premium feature, so instead we will use the “Manually Trigger a Flow” trigger.

The trigger for our Microsoft Teams flow

This trigger requires authentication, but because it is assumed that the attacker is part of the organization this shouldn’t be a problem, and there are ways to limit information about who is running what flows.

Creating the flow logic

Flows allow you to create an automated process piece by piece, passing the outputs of one action to the next. For example, in the flow we created to let us get all Teams messages from a user, as well as send messages to any channel on their behalf, we determine what action to take, who to send the message to and other details depending on the input passed to the trigger.

Sending a message is quick and simple, but to retrieve all messages for all teams and channels, we first grab a list of all teams, then get each channel per team, then all messages per channel, and roll it up into one big gross ball and have the flow send it to the attacker via email.

The Teams flow for our PoC

Now that we have the flow created, we need to know how we can create it, and share it with ourselves as the attacker, using the tokens we’ve stolen and what those requests look like. Luckily in our case, it is just a couple of simple requests.

  1. A POST request, containing JSON object representing the flow, to create it and get the unique flow name.
  2. A GET request to grab the flow trigger uri, which will let us trigger the flow as the attacker once we have added ourselves to the owners group.

Adding a group to flow owners

For the trigger we chose, we need to be able to access the flow trigger uri, which can only be done by users who have access to the flow. As a result, we need to add a group we belong to (which seems less suspicious than just adding ourselves) to the flow owners.

The easiest choice here is some large, all-encompassing group, but in our case we’re using the group which is generated automatically for any team created in Microsoft Teams.

In order to grab the unique group id, we use the graph.microsoft.com token we stole from the victim earlier. We then modify the flow’s owners to include that group.

adding a group to the flow owners

Running the flow and sending ourselves the uris we need

In the proof of concept we’re building, we create a flow that lets us send emails on behalf of the victim user. This can be leveraged at the end of the attack to send ourselves the list of the flow trigger uris we need in order to perform the actions we want.

sending an email using the Outlook connector and flow we’ve created

For example, at the end of the email/Teams proof of concept we’re building, an email is sent on the victim’s behalf which sends us the flow trigger uris for both the Outlook and Teams flows we’ve created.

The message we receive from the victim with the flow trigger uris

Using these flow trigger uris, we can now read the victim’s emails and Teams messages, and send messages and emails on their behalf (despite being authenticated as Bad User).

Putting it all together

The “TL;DR” shot: actions the malicious tab performs on opening

There are a number of ways in which we could build an attack with this vulnerability. It is likely that the best way would be to only use javascript on the malicious tab to steal the service.flow.microsoft.com token, and then perform the rest of the actions from an attacker-controlled server, so we reduce the traffic being generated by the victims and aren’t cut off by them navigating away from the tab.

For our quick and dirty PoC however, we just perform the whole attack with one big javascript section in our malicious tab. The pseudocode for our attack looks like this:

Setting up a malicious tab with a payload like the one above will cause the victim to create connectors and flows, and add the attacker as an owner to them, as well as send them an email containing the flow trigger uris.

As a real example, here is a quick clip of a similar payload running and sending the attacker the victim’s Teams messages, and letting the attacker send a message to a private team masquerading as the victim.

stealing and sending Teams messages

Considerations for the attacker

If you’ve gone through the above and thought “cool, but it would be really easy for an admin to determine who is using these flows maliciously,” you’d be correct. However, there are a number of steps one could take to limit the exposure of the attacking user if a similar attack is being carried out in a penetration test.

  • Flows allow you to specify whether the inputs and outputs to each action should be kept secret / scrubbed from the flow’s run history. This means that it would be harder to observe what data is being taken, and where it is being sent.
  • Not all flows require the user to make authenticated requests to trigger. Low and slow methods like having flows trigger on a RSS feed update (30 minute minimum period), or on a schedule, or automatically (like when a new email comes in from any account, read the email body and perform those actions).
  • Running the attack as one long javascript payload isn’t ideal and takes too long in real situations. Just grabbing the service.flow.microsoft.com token and conducting the rest of the attack from an attacker-controlled machine would be much less conspicuous.
  • Flows can be used to creatively cover an attacker’s tracks. For example, if you exfiltrate data via email in a flow, you can add a final step which deletes any emails sent to the attacker’s mail from the Sent Items folder.

Considerations for org administrators

While it may be difficult to determine who in a team has set up a malicious tab, or what user is running the flows (if the inputs/outputs have been made secret), there is a potential indicator to identify whether a user has had malicious flows run on their behalf.

When a user logs into make.powerapps.com or flow.microsoft.com to create a flow, a Microsoft Power Automate free license is automatically added to their set of licenses (if they didn’t already have one assigned to them). However, when flows are created on a user’s behalf by a malicious tab, they don’t have the license assigned to them. This license status can be cross referenced with which users have flows created under their name at admin.powerplatform.microsoft.com

organization admin portal

Notice that Bad User has logged into the flow.microsoft.com web interface, but Good User, despite having flows in their name listed in admin.powerplatform.microsoft.com, does not show as having a license for Power Automate. This could indicate that the flows were not created intentionally by Good User.

Luckily, the attack is limited to authenticated users within a Teams organization who have the ability to create Power Apps tabs, which means it can’t just be exploited by an untrusted/unauthenticated attacker. However, the permission to create these tabs is enabled by default, so it may be a good idea to consider limiting apps by default and enable them on request.

Takeaways

While that was a long and not quite straightforward attack, the potential impact of such an attack could be huge, especially if it happens to hit an organization administrator. That such a small initial bug (the improper validation of the make.powerapps.com domain) could be traded-up until an attacker is exfiltrating emails, Teams messages, OneDrive and SharePoint files is definitely concerning. It means that even a small bug in a not-so-common service like Microsoft Power Apps could lead to the compromise of many other services by way of token bundles and first party logins for connectors.

So if you happen to find a small bug in one service, see how far you can take it and see if you can trade a small bug for a big impact. There are likely other creative and serious potential attacks we didn’t explore with all of the potential access tokens we were able to steal. Let me know if you spot one 🙂.

Thanks for reading!


Stealing tokens, emails, files and more in Microsoft Teams through malicious tabs 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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