CVE-2021-31166: HTTP Protocol Stack Remote Code Execution Vulnerability

This is a proof of concept for CVE-2021-31166 (“HTTP Protocol Stack Remote Code Execution Vulnerability”), a use-after-free dereference in http.sys patched by Microsoft in May 2021. According to this tweet the vulnerability has been found by @_mxms and @fzzyhd1.


The bug itself happens in http!UlpParseContentCoding where the function has a local LIST_ENTRY and appends item to it. When it’s done, it moves it into the Request structure; but it doesn’t NULL out the local list. The issue with that is that an attacker can trigger a code-path that frees every entries of the local list leaving them dangling in the Request object.


Here is the bugcheck:

KDTARGET: Refreshing KD connection

*** Fatal System Error: 0x00000139

Break instruction exception - code 80000003 (first chance)

A fatal system error has occurred.
Debugger entered on first try; Bugcheck callbacks have not been invoked.

A fatal system error has occurred.

fffff804`19410c50 cc              int     3

kd> kp
 # Child-SP          RetAddr               Call Site
00 fffff90e`a867e368 fffff804`19525382     nt!DbgBreakPointWithStatus
01 fffff90e`a867e370 fffff804`19524966     nt!KiBugCheckDebugBreak+0x12
02 fffff90e`a867e3d0 fffff804`19408eb7     nt!KeBugCheck2+0x946
03 fffff90e`a867eae0 fffff804`1941ad69     nt!KeBugCheckEx+0x107
04 fffff90e`a867eb20 fffff804`1941b190     nt!KiBugCheckDispatch+0x69
05 fffff90e`a867ec60 fffff804`19419523     nt!KiFastFailDispatch+0xd0
06 fffff90e`a867ee40 fffff804`1db3f677     nt!KiRaiseSecurityCheckFailure+0x323
07 fffff90e`a867efd0 fffff804`1daf6c05     HTTP!UlFreeUnknownCodingList+0x63
08 fffff90e`a867f000 fffff804`1dacd201     HTTP!UlpParseAcceptEncoding+0x299c5
09 fffff90e`a867f0f0 fffff804`1daa93d8     HTTP!UlAcceptEncodingHeaderHandler+0x51
0a fffff90e`a867f140 fffff804`1daa8ab7     HTTP!UlParseHeader+0x218
0b fffff90e`a867f240 fffff804`1da04c5f     HTTP!UlParseHttp+0xac7
0c fffff90e`a867f3a0 fffff804`1da0490a     HTTP!UlpParseNextRequest+0x1ff
0d fffff90e`a867f4a0 fffff804`1daa48c2     HTTP!UlpHandleRequest+0x1aa
0e fffff90e`a867f540 fffff804`1932ae85     HTTP!UlpThreadPoolWorker+0x112
0f fffff90e`a867f5d0 fffff804`19410408     nt!PspSystemThreadStartup+0x55
10 fffff90e`a867f620 00000000`00000000     nt!KiStartSystemThread+0x28

kd> !analyze -v
*                                                                             *
*                        Bugcheck Analysis                                    *
*                                                                             *

A kernel component has corrupted a critical data structure.  The corruption
could potentially allow a malicious user to gain control of this machine.
Arg1: 0000000000000003, A LIST_ENTRY has been corrupted (i.e. double remove).
Arg2: fffff90ea867ee40, Address of the trap frame for the exception that caused the BugCheck
Arg3: fffff90ea867ed98, Address of the exception record for the exception that caused the BugCheck
Arg4: 0000000000000000, Reserved

Frequently Asked Questions

Q: Is Windows Remote Management (WinRM) affected?

Yes (thanks to @JimDinMN for sharing his experiments).

Q: Is Web Services on Devices (WSDAPI) affected?

Yes (thanks to @HenkPoley for sharing his results).

Q: What are the affected versions of Windows?

According to Microsoft’s documentation, here are the affected platforms:

  • Windows Server, version 2004 (or 20H1) (Server Core installation),
  • Windows 10 Version 2004 (or 20H1) for ARM64/x64/32-bit Systems,
  • Windows Server, version 20H2 (Server Core Installation),
  • Windows 10 Version 20H2 for ARM64/x64/32-bit Systems.


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