Amsi Bypass on Windows 11 In 2023

Technical details can be found in the article: https://medium.com/@gustavshen/bypass-amsi-on-windows-11-75d231b2cac6

Attack_AmsiOpenSession.ps1

HRESULT AmsiOpenSession(
[in] HAMSICONTEXT amsiContext,
[out] HAMSISESSION *amsiSession
);

This powershell script can be used to bypass AMSI by patching AmsiOpenSession. According to the assemble codes, if any of the following conditions are met, the function will exit with E_INVALIDARG error.

1. RCX is 0
2. RDX is 0
3. The 2nd QWORD of HAMSICONTEXT structure is 0
4. The 3rd QWORD of HAMSICONTEXT structure is 0

This script patches AmsiOpenSession by setting RCX to 0.

Patching AmsiOpenSession cannot bypass AMSI for Assembly.Load()

Attack_AmsiScanBuffer.ps1

HRESULT AmsiScanBuffer(
[in] HAMSICONTEXT amsiContext,
[in] PVOID buffer,
[in] ULONG length,
[in] LPCWSTR contentName,
[in, optional] HAMSISESSION amsiSession,
[out] AMSI_RESULT *result
);