Patch diffing CVE-2022–21907

Adversary Academy Research
5 min readJan 12, 2022


Our security team does an in-depth analysis of critical security vulnerabilities when they are released on patch Tuesday. This patch Tuesday one interesting bug caught our eye. CVE-2022–21907 HTTP Protocol Stack Remote Code Execution Vulnerability, reading through the description words like critical, wormable, etc caught my interest. So we began with a differential analysis of the patch. FYI this story will be updated as I progress with static and dynamic analysis, some assumptions on root cause will most likely be wrong and will be updated as progress is made.

After backing up the December version of http.sys I installed the patch on an analysis machine and performed a differential analysis using IDA pro and BinDiff. There were only a few updated function names in the patched binary.

Only a few changed functions

The updated functions in the binary are UlFastSendHttpResponse with roughly 10% changed across the patch (that's a lot), UlpAllocateFastTracker UlpFastSendCompleteWorker UlpFreeFastTracker and UlAllocateFastTrackerToLookaside. Just reviewing the naming convention of the functions makes me think “use after free” due to the functions handling some sort of allocations, and free’s namely UlpAllocate* and UlpFreeFastTracker. The naming convention makes me think these functions are allocating and freeing chunks of memory.

Without any particular approach to targeting patched functions, let's begin with a review of the basic blocks in UlpFreeFastTracker.

UlpFreeFastTracker Unpatched (on the left) And patched on the right

We can see in UlpFreeFastTracker after returning from a call into UlDestroyLogDataBuffer the unpatched function does nothing before jumping to the next basic block. The patched function on the right ANDs the values in [rbx+0xb0] with 0. Not entirely sure of the reasoning behind that but runtime debugging or further reversing of UlpFreeFastTracker may help.

Another interesting function with a number of changes is UlPAllocateFastTracker. In the patched version, there are a number of changed basic blocks. Changes that stand out are the multiple calls to memset in order to zero out memory. This is one way to squash memory corruption bugs, so our theory is looking good.

memset is added

memset is called again on another basic block before a call to UxDuoIniutializeCollection. UxDuoInitializeCollection is also setting memory to 0 memset at an arbitrary size of 138 bytes. This is unchanged from the previous version so probably not the issue.

additional memset of 0

What is interesting about the first memset in this function is it's an arbitrary size and not a dynamic size. Maybe this is trying to fix something? However, since it's not a dynamic size, maybe there is still space for use after free in other size chunks? or maybe all chunks in this bug are a static size. Just a theory at this point.

memset 0 on 290 byte buffer at rax

Proceeding to the function with the most changes UlFastSendHttpResponse this function is by far more complex than the others. I miss those patch diffing examples with 3 lines of assembly code.

Looking at all of the changes in UlFastSendHttpResponse was a little complex and I’m still trying to understand what it does. However, we can see that the code from UlFastSendHttpResponse does reach UlpFreeFastTracker

There is a call to UlpFreeFastTracker from UlfastSendHttpResponse

Further analysis reveals that there is also a call into UlpAllocateFastTracker.

Direct path into UlpAllocateFastTracker

At this point, a safe assumption may be that the vulnerable code path is hit first in UlFastSendHttpResponse and some of the fixup / mitigations were applied to memory chunks in the other functions. We need to know how to reach the UlFastSendHttpResponse. The only insight that Microsoft gives us is that registry-based mitigations will disable trailer support.


The enableTrailerSupport registry key should be set to 0 to mitigate the risk, or in our case, it should be enabled and we can check code paths that are hit when we make web requests that include a trailer parameter.

Trailers are defined in RFC7230, more details here

Update as of 1/13/22

The next step would be to make requests that include the trailer parameter and record code paths/code coverage and see if it's possible to get close to the patched code with relative ease. For those that are following along the approach, I plan to take is to fuzz HTTP requests with chunked transfer encoding. I’ll post the results back here but an example to use to start building a corpus would look like this

In the meantime, another researcher on attackerkb shared the text of a kernel bugcheck after a crash. The bugcheck states that a stack overflow was potentially detected in UlFreeUnknownCodingList. Below is the path that the patched function UlFastSendHTTPResponse can take to reach UlFreeUnknownCodingList via UlpFreeHttpRequest. It seems as if we are on the right path.

This looks promising

Update 1/19/22

I had some issues with my target VM patching itself (thanks Microsoft) I’ve reinstalled a fresh windows 10 install and I’m currently fuzzing HTTP chunked requests with Radamsa. I’ll post the sample here when I trigger a crash.

Update 1/20/22

There’s been some confusion lately, a few other researchers have posted exploits related to CVE-2021–31666 and not affecting patched (December) versions of Windows 10 21H2 and 1809 at least. I haven’t seen a single exploit that targets the Transfer-Encoding & chunked requests as specified in the CVE. However, it does appear that those call stacks and bugs are closely related in the code of http.sys. The close-in-nature relation may be the cause of the confusion. I’d recommend reading for details on that bug. It’s also possible to validate that this bug is different due to the fact that the December vs January patch of http.sys does not include any changes to the vulnerable code path in cve-2021–3166. For cve-2021–3166 affected functions are UlAcceptEncodingHeaderHandler, UlpParseAcceptEncoding, and UlpParseContentCoding respectively.