Potential AMSI Bypass Script Using NULL Bits — Sigma Rule

Rule metadata and linked tags

Author: Nasreddine Bencherchali (Nextron Systems). Source status: test; severity: medium. Source dates: 2023-01-04 / 2023-05-09.

{
  "product": "windows",
  "category": "ps_script",
  "definition": "Requirements: Script Block Logging must be enabled"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 00e5953d97cf9c2c349f2421a6b8073cc2ae356e25df46305a4e50b3b6df8eb6

Detection logic

Original source YAML. Source-tag agreement is not proof of complete semantic coverage. This rule has not been compiled for a SIEM backend or validated against live telemetry here.

title: Potential AMSI Bypass Script Using NULL Bits
id: fa2559c8-1197-471d-9cdd-05a0273d4522
related:
    - id: 92a974db-ab84-457f-9ec0-55db83d7a825
      type: similar
status: test
description: Detects usage of special strings/null bits in order to potentially bypass AMSI functionalities
references:
    - https://github.com/r00t-3xp10it/hacking-material-books/blob/43cb1e1932c16ff1f58b755bc9ab6b096046853f/obfuscation/simple_obfuscation.md#amsi-bypass-using-null-bits-satoshi
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-01-04
modified: 2023-05-09
tags:
    - attack.defense-impairment
    - attack.t1685
logsource:
    product: windows
    category: ps_script
    definition: 'Requirements: Script Block Logging must be enabled'
detection:
    selection:
        ScriptBlockText|contains:
            - "if(0){{{0}}}' -f $(0 -as [char]) +"
            - "#<NULL>"
    condition: selection
falsepositives:
    - Unknown
level: medium

Original YAML and metadata in JSON

False positives

• Unknown

Source references

• https://github.com/r00t-3xp10it/hacking-material-books/blob/43cb1e1932c16ff1f58b755bc9ab6b096046853f/obfuscation/simple_obfuscation.md#amsi-bypass-using-null-bits-satoshi