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Detection rules › Sigma

AMSI Bypass Pattern Assembly GetType

Status
test
Severity
high
Log source
product windows, category ps_script
Author
Florian Roth (Nextron Systems)
Source
github.com/SigmaHQ/sigma

Detects code fragments found in small and obfuscated AMSI bypass PowerShell scripts

MITRE ATT&CK coverage

TacticTechniques
Defense ImpairmentT1685 Disable or Modify Tools

Event coverage

ProviderEventTitle
PowerShellEvent ID 4104Creating Scriptblock text (MessageNumber of MessageTotal).

Rule body yaml

title: AMSI Bypass Pattern Assembly GetType
id: e0d6c087-2d1c-47fd-8799-3904103c5a98
status: test
description: Detects code fragments found in small and obfuscated AMSI bypass PowerShell scripts
references:
    - https://www.mdsec.co.uk/2018/06/exploring-powershell-amsi-and-logging-evasion/
    - https://twitter.com/cyb3rops/status/1588574518057979905?s=20&t=A7hh93ONM7ni1Rj1jO5OaA
author: Florian Roth (Nextron Systems)
date: 2022-11-09
tags:
    - attack.defense-impairment
    - attack.t1685
    - attack.execution
logsource:
    product: windows
    category: ps_script
    definition: 'Requirements: Script Block Logging must be enabled'
detection:
    selection:
        ScriptBlockText|contains|all:
            - '[Ref].Assembly.GetType'
            - 'SetValue($null,$true)'
            - 'NonPublic,Static'
    condition: selection
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: condition

selection

Stage 1: selection

selection:
    ScriptBlockText|contains|all:
        - '[Ref].Assembly.GetType'
        - 'SetValue($null,$true)'
        - 'NonPublic,Static'

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

FieldKindValues
ScriptBlockTextmatch
  • NonPublic,Static
  • SetValue($null,$true)
  • [Ref].Assembly.GetType