AMSI_VEH: A Powershell AMSI Bypass technique via Vectored Exception Handler

AMSI Bypass via VEH

A PowerShell AMSI Bypass technique via Vectored Exception Handler (VEH). This technique does not perform assembly instruction patching, function hooking or Import Address Table (IAT) modification.

How it works:

For this technique to work, you must first inject the VEH DLL into the PowerShell process. This can be done either by injecting the DLL or via DLL hijacking .

This technique works by setting up a hardware breakpoint on the function AmsiScanBuffer on all PowerShell process threads, then installing a VEH to handle the trigger of this breakpoint.

When a thread calls AmsiScanBuffer, the VEH will make the thread to exit the function without executing anything and setting the result of the function to AMSI_RESULT_CLEAN. This is all done inside the VEH, without modifying the code of the process or without any PE modifications.

Use

For demonstration purposes, this repository contains a very basic DLL injector. Use it this way:

• 1.) Compile the DLL Injector and VEH DLL.
• 2.) Open an instance of PowerShell.
• 3.) Run the DLL injector by providing the FULL PATH to the DLL. Example:

• 4.) PowerShell will open a MessageBox window to confirm the VEH installation.

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