AMSI patch → in-memory .NET / PowerShell stager

§ Context

Assumed environment: medium-integrity foothold. Attacker has the ability to start a PowerShell session or load .NET assembly in their own process. AV relies on AMSI for in-memory script scanning.

§ Steps

1. 01
   Medium-integrity shellInitial Access

   T1078— Valid Accounts
2. 02
   C2 beacon establishedCommand and Control

   T1071— Application Layer Protocol
3. 03
   Persistence via WMI subscriptionPersistence

   LOL-SCRCONS— scrcons.exe WMI Event Subscription
4. 04
   Patch amsi.dll!AmsiScanBufferDefense Evasion

   AMSI-PATCH— AMSI In-Memory Patch
5. 05
   Patch ntdll!EtwEventWrite to blind ETWDefense Evasion

   ETW-PATCH— ETW Event-Tracing Patch
6. 06
   Load .NET stager via Assembly.LoadDefense Evasion

   INJ-MODULE-STOMP— Module Stomping

§ References

• T1078Valid Accounts
• T1071Application Layer Protocol

§ Frequently asked

What is the "AMSI patch → in-memory .NET / PowerShell stager" attack path?

Patch AmsiScanBuffer in amsi.dll → return clean for any content. Subsequent PowerShell / Office VBA / .NET runtime calls emit attacker code without scanning. It chains 6 steps drawn from real-world offensive-security techniques.

What starting position does this attack require?

The first step is Medium-integrity shell (T1078) — a initial access primitive. Assumed environment: medium-integrity foothold.

What is the final impact of this kill-chain?

The final step lands on Load .NET stager via Assembly.Load (INJ-MODULE-STOMP), which falls under Defense Evasion. From here, an operator typically pivots into post-exploitation or maintains persistence.

How can defenders detect or prevent this attack?

Detection and prevention vary per step. Refer to each linked MITRE ATT&CK entry under "References" — every technique on that page lists defensive controls, detection telemetry, and known threat-actor usage.

§ Related dossiers

• Shared techniques2

  Build-system implant → signed supply-chain backdoor (SolarWinds-class)

  Compromise the target vendor's build server. A small implant rewrites a single source file at compile time — every official signed release downstream now ships the backdoor.

• Shared techniques2

  Process hollowing → run beacon in svchost shell

  Spawn svchost.exe suspended, unmap its image, write attacker PE into the same address space, resume — the process keeps a legit-looking PEB and command line but executes beacon code.

• Shared techniques2

  certutil + bitsadmin → AV-friendly stager chain

  Initial access dropped a tiny .bat. It uses certutil to decode a base64 blob and bitsadmin to fetch the real beacon, then schtasks for persistence. Every binary is signed Microsoft.

• Shared techniques2

  Autodiscover external leak → credential harvest

  Mis-implemented Autodiscover falls back to autodiscover.<TLD>; register that domain externally, harvest plaintext Basic-auth credentials from clients that haven't been patched / configured properly.