Potential AMSI COM Server Hijacking

Description

Detects changes to the AMSI come server registry key in order disable AMSI scanning functionalities. When AMSI attempts to starts its COM component, it will query its registered CLSID and return a non-existent COM server. This causes a load failure and prevents any scanning methods from being accessed, ultimately rendering AMSI useless

Query · sigma

selection:
  TargetObject|endswith: \CLSID\{fdb00e52-a214-4aa1-8fba-4357bb0072ec}\InProcServer32\(Default)
filter:
  Details: '%windir%\system32\amsi.dll'
condition: selection and not filter

Known false positives

  • Unknown
Raw source Potential AMSI COM Server Hijacking · Sigma
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Published by SigmaHQ/sigma ↗, licensed under Detection Rule License 1.1 ↗. Reproduced here unmodified. 
title: Potential AMSI COM Server Hijacking
id: 160d2780-31f7-4922-8b3a-efce30e63e96
status: test
description: Detects changes to the AMSI come server registry key in order disable AMSI scanning functionalities. When AMSI attempts to starts its COM component, it will query its registered CLSID and return a non-existent COM server. This causes a load failure and prevents any scanning methods from being accessed, ultimately rendering AMSI useless
references:
    - https://enigma0x3.net/2017/07/19/bypassing-amsi-via-com-server-hijacking/
    - https://github.com/r00t-3xp10it/hacking-material-books/blob/43cb1e1932c16ff1f58b755bc9ab6b096046853f/obfuscation/simple_obfuscation.md#amsi-comreg-bypass
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-01-04
modified: 2023-08-17
tags:
    - attack.defense-impairment
    - attack.t1685
logsource:
    category: registry_set
    product: windows
detection:
    selection:
        TargetObject|endswith: '\CLSID\{fdb00e52-a214-4aa1-8fba-4357bb0072ec}\InProcServer32\(Default)'
    filter:
        Details: '%windir%\system32\amsi.dll'
    condition: selection and not filter
falsepositives:
    - Unknown
level: high

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