Suspicious FromBase64String Usage On Gzip Archive - Ps Script — Sigma Rule

Rule metadata and linked tags

Author: frack113. Source status: test; severity: medium. Source dates: 2022-12-23 / not supplied.

{
  "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: 9f64b371a6893f74354957e4746265bec44550e0b119d5c2ef380668c847efb3

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: Suspicious FromBase64String Usage On Gzip Archive - Ps Script
id: df69cb1d-b891-4cd9-90c7-d617d90100ce
related:
    - id: d75d6b6b-adb9-48f7-824b-ac2e786efe1f
      type: similar
status: test
description: Detects attempts of decoding a base64 Gzip archive in a PowerShell script. This technique is often used as a method to load malicious content into memory afterward.
references:
    - https://speakerdeck.com/heirhabarov/hunting-for-powershell-abuse?slide=43
author: frack113
date: 2022-12-23
tags:
    - attack.command-and-control
    - attack.t1132.001
logsource:
    product: windows
    category: ps_script
    definition: 'Requirements: Script Block Logging must be enabled'
detection:
    selection:
        ScriptBlockText|contains|all:
            - 'FromBase64String'
            - 'MemoryStream'
            - 'H4sI'
    condition: selection
falsepositives:
    - Legitimate administrative script
level: medium

Original YAML and metadata in JSON

False positives

• Legitimate administrative script

Source references

• https://speakerdeck.com/heirhabarov/hunting-for-powershell-abuse?slide=43