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Shadow Stack Walk

Gabriel Landau @ Elastic Security

See the accompanying Elastic Security Labs article, Finding Truth in the Shadows.

The shadow stack provides an interesting detection opportunity. Adversaries can use tools like ThreadStackSpoofer and CallStackSpoofer to obfuscate their presence against thread stack scans (e.g. StackWalk64) and inline stack traces like Sysmon operations.

By comparing a traditional stack walk against its shadowy sibling, we can both detect and see through thread stack spoofing. This tool implements CaptureStackBackTrace/StackWalk64 with the shadow stack (aka CET/HSP) to catch thread stack spoofing. When the stack is normal, it functions similarly to CaptureStackBackTrace and StackWalk64:

Control run demonstrating equivalent output...

CONTROL CaptureStackBackTrace: dps 000001CE41EDA800
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!main + 0x5f
1: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__scrt_common_main_seh + 0x10c
2: \Device\HarddiskVolume3\Windows\System32\kernel32.dll!BaseThreadInitThunk + 0x14
3: \Device\HarddiskVolume3\Windows\System32\ntdll.dll!RtlUserThreadStart + 0x21

CONTROL StackWalk64: dps 000001CE42F80D80
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!main + 0x5f
1: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__scrt_common_main_seh + 0x10c
2: \Device\HarddiskVolume3\Windows\System32\kernel32.dll!BaseThreadInitThunk + 0x14
3: \Device\HarddiskVolume3\Windows\System32\ntdll.dll!RtlUserThreadStart + 0x21

CONTROL CET Stack: dps 000001CE41ED79C0
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!main + 0x5f
1: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__scrt_common_main_seh + 0x10c
2: \Device\HarddiskVolume3\Windows\System32\kernel32.dll!BaseThreadInitThunk + 0x14
3: \Device\HarddiskVolume3\Windows\System32\ntdll.dll!RtlUserThreadStart + 0x21

It's unaffected by intentional breaks of the call stack such as ThreadStackSpoofer.

Breaking stack walk with a NULL return address...

BROKEN CaptureStackBackTrace: dps 000001CE41ED79C0
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!SpoofStackThenCall + 0x40

BROKEN StackWalk64: dps 000001CE42E1FCD0
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!SpoofStackThenCall + 0x40

BROKEN CET Stack: dps 000001CE41F23020
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!SpoofStackThenCall + 0x40
1: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!main + 0x8c
2: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__scrt_common_main_seh + 0x10c
3: \Device\HarddiskVolume3\Windows\System32\kernel32.dll!BaseThreadInitThunk + 0x14
4: \Device\HarddiskVolume3\Windows\System32\ntdll.dll!RtlUserThreadStart + 0x21

It doesn't care about forged stack frames:

Spoofing call stack to hide ShadowStackWalk.exe!main...

SPOOFED CaptureStackBackTrace: dps 000001CE41ED79C0
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!SpoofStackThenCall + 0x40
1: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__scrt_common_main_seh + 0x10c
2: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__xi_z + 0x0

SPOOFED StackWalk64: dps 000001CE41F23020
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!SpoofStackThenCall + 0x40
1: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__scrt_common_main_seh + 0x10c
2: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__xi_z + 0x0

SPOOFED CET Stack: dps 000001CE42C927C0
0: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!SpoofStackThenCall + 0x40
1: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!main + 0xbe
2: \Device\HarddiskVolume3\git\ShadowStackWalk\x64\Release\ShadowStackWalk.exe!__scrt_common_main_seh + 0x10c
3: \Device\HarddiskVolume3\Windows\System32\kernel32.dll!BaseThreadInitThunk + 0x14
4: \Device\HarddiskVolume3\Windows\System32\ntdll.dll!RtlUserThreadStart + 0x21