Windows: .NET Reflection Attempt to Disable AMSI via amsiInitFailed

ATT&CK techniques

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Resource Dev
Initial Access
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What it detects

This rule flags process creation events where the command line references PowerShell AMSI utilities and specifically the string "amsiInitFailed". It also matches .NET reflection usage patterns that set a non-public static value to $null/$true, which can be used to impair AMSI scanning. The detection relies on Windows process command-line telemetry and string matches within the PowerShell/.NET invocation.

Reporting behind it

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Windows: .NET Reflection Attempt to Disable AMSI via amsiInitFailed

Pivot detection · T1685 · 9 related

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.

windows-net-reflection-attempt-to-disable-amsi-via-amsiinitfailed-30edb182

title: "Windows: .NET Reflection Attempt to Disable AMSI via amsiInitFailed"
id: 60606a36-7f20-4afc-bf0f-379987c1ada3
related:
  - id: 4f927692-68b5-4267-871b-073c45f4f6fe
    type: obsolete
  - id: 30edb182-aa75-42c0-b0a9-e998bb29067c
    type: derived
status: test
description: This rule flags process creation events where the command line references PowerShell AMSI utilities and specifically the string "amsiInitFailed". It also matches .NET reflection usage patterns that set a non-public static value to $null/$true, which can be used to impair AMSI scanning. The detection relies on Windows process command-line telemetry and string matches within the PowerShell/.NET invocation.
references:
  - https://s3cur3th1ssh1t.github.io/Bypass_AMSI_by_manual_modification/
  - https://www.mdsec.co.uk/2018/06/exploring-powershell-amsi-and-logging-evasion/
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_powershell_amsi_init_failed_bypass.yml
author: Markus Neis, @Kostastsale, Huntrule Team
date: 2018-08-17
modified: 2023-02-03
tags:
  - attack.defense-impairment
  - attack.t1685
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    - CommandLine|contains|all:
        - System.Management.Automation.AmsiUtils
        - amsiInitFailed
    - CommandLine|contains|all:
        - "[Ref].Assembly.GetType"
        - SetValue($null,$true)
        - NonPublic,Static
  condition: selection
falsepositives:
  - Unlikely
level: high
license: DRL-1.1

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