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Potential AMSI Bypass via RPC Runtime Hooking

Identifies PowerShell script block content associated with an Antimalware Scan Interface (AMSI) bypass that hooks the RPC runtime marshaling stub NdrClientCall3 (or NdrClientCall2) in rpcrt4.dll. Unlike bypasses that…

Description

Identifies PowerShell script block content associated with an Antimalware Scan Interface (AMSI) bypass that hooks the RPC runtime marshaling stub NdrClientCall3 (or NdrClientCall2) in rpcrt4.dll. Unlike bypasses that patch AmsiScanBuffer or set amsiInitFailed, this technique operates at the RPC layer used by AMSI to delegate scan requests to the antivirus provider, tampering with the request before it reaches the engine and leaving AMSI itself unmodified. The loader allocates an executable trampoline and marshals a delegate to the native stub; these primitives appear in PowerShell Script Block Logging before the hook takes effect.

Detection logic

Its licence does not clear it for publishing here

Sunturai publishes a detection's own text where the licence it arrived under has been reviewed and permits it, and Elastic License 2.0 has not. The query as its source wrote it, its canonical form and the hash that pins this revision are in the workspace record.

Detection requirements

Platform
ContainersESXiIaaSIdentity ProviderLinuxmacOSNetwork DevicesOffice SuiteSaaSWindows

The rule states no platform. This is derived from the 2 ATT&CK techniques it maps to.

Log source category
event_index

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