Challenge 31: AMSI Initialization Failure

Difficulty: Medium
Category: AMSI Bypass

Objective

Convince PowerShell that AMSI failed to initialize, causing it to skip all subsequent AMSI scans for the current session. Execute a payload containing known-bad signatures without triggering the AMSI provider.

Scanner Behavior

AMSI (Antimalware Scan Interface) is a Windows API that allows applications like PowerShell to submit content to registered antimalware providers for scanning. When you type a command in PowerShell or run a script, the content is sent via AmsiScanBuffer to our custom AMSI provider DLL (nim_amsi_wrapper_dll), which checks it against known signatures.

PowerShell’s AMSI integration is implemented in .NET managed code. Internally, it tracks whether AMSI initialized successfully using a private static field. If this field indicates that initialization failed, PowerShell skips all AMSI scans entirely — it never calls AmsiScanBuffer at all.

This means the bypass happens at the application layer (PowerShell), not the AMSI provider layer. The provider never even sees the content.

Rules

You must modify the internal state of the current PowerShell process to disable AMSI.
After your bypass, subsequent commands containing signatures like malware or Invoke-Mimikatz must not be flagged.
You may not kill or restart the PowerShell process.
You may not modify amsi.dll on disk.

Hints

.NET Reflection allows you to access private and internal members of classes, even static fields that are not meant to be exposed.
The relevant class lives in the System.Management.Automation namespace. It is a utility class related to AMSI.
The field name is descriptive — it literally says what happened to initialization.
Setting a boolean to $true is all it takes once you find the right target.

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