PowerShell Base64 Encoded Reflective Assembly Load — Sigma Rule

Rule metadata and linked tags

Author: Christian Burkard (Nextron Systems), pH-T (Nextron Systems). Source status: test; severity: high. Source dates: 2022-03-01 / 2023-01-30.

{
  "category": "process_creation",
  "product": "windows"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: e21ad44fa75e14737ee876428435d8fe5f2be5eeba12219ecef79baa54ea1756

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: PowerShell Base64 Encoded Reflective Assembly Load
id: 62b7ccc9-23b4-471e-aa15-6da3663c4d59
related:
    - id: 9c0295ce-d60d-40bd-bd74-84673b7592b1
      type: similar
status: test
description: Detects base64 encoded .NET reflective loading of Assembly
references:
    - https://github.com/Neo23x0/Raccine/blob/20a569fa21625086433dcce8bb2765d0ea08dcb6/yara/mal_revil.yar
    - https://thedfirreport.com/2022/05/09/seo-poisoning-a-gootloader-story/
author: Christian Burkard (Nextron Systems), pH-T (Nextron Systems)
date: 2022-03-01
modified: 2023-01-30
tags:
    - attack.execution
    - attack.stealth
    - attack.t1059.001
    - attack.t1027
    - attack.t1620
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        CommandLine|contains:
            # [Reflection.Assembly]::Load(
            - 'WwBSAGUAZgBsAGUAYwB0AGkAbwBuAC4AQQBzAHMAZQBtAGIAbAB5AF0AOgA6AEwAbwBhAGQAKA'
            - 'sAUgBlAGYAbABlAGMAdABpAG8AbgAuAEEAcwBzAGUAbQBiAGwAeQBdADoAOgBMAG8AYQBkACgA'
            - 'bAFIAZQBmAGwAZQBjAHQAaQBvAG4ALgBBAHMAcwBlAG0AYgBsAHkAXQA6ADoATABvAGEAZAAoA'
            # [reflection.assembly]::("Load")
            - 'AFsAcgBlAGYAbABlAGMAdABpAG8AbgAuAGEAcwBzAGUAbQBiAGwAeQBdADoAOgAoACIATABvAGEAZAAiAC'
            - 'BbAHIAZQBmAGwAZQBjAHQAaQBvAG4ALgBhAHMAcwBlAG0AYgBsAHkAXQA6ADoAKAAiAEwAbwBhAGQAIgAp'
            - 'AWwByAGUAZgBsAGUAYwB0AGkAbwBuAC4AYQBzAHMAZQBtAGIAbAB5AF0AOgA6ACgAIgBMAG8AYQBkACIAK'
            # [Reflection.Assembly]::("Load")
            - 'WwBSAGUAZgBsAGUAYwB0AGkAbwBuAC4AQQBzAHMAZQBtAGIAbAB5AF0AOgA6ACgAIgBMAG8AYQBkACIAKQ'
            - 'sAUgBlAGYAbABlAGMAdABpAG8AbgAuAEEAcwBzAGUAbQBiAGwAeQBdADoAOgAoACIATABvAGEAZAAiACkA'
            - 'bAFIAZQBmAGwAZQBjAHQAaQBvAG4ALgBBAHMAcwBlAG0AYgBsAHkAXQA6ADoAKAAiAEwAbwBhAGQAIgApA'
            # [reflection.assembly]::Load(
            - 'WwByAGUAZgBsAGUAYwB0AGkAbwBuAC4AYQBzAHMAZQBtAGIAbAB5AF0AOgA6AEwAbwBhAGQAKA'
            - 'sAcgBlAGYAbABlAGMAdABpAG8AbgAuAGEAcwBzAGUAbQBiAGwAeQBdADoAOgBMAG8AYQBkACgA'
            - 'bAHIAZQBmAGwAZQBjAHQAaQBvAG4ALgBhAHMAcwBlAG0AYgBsAHkAXQA6ADoATABvAGEAZAAoA'
    condition: selection
falsepositives:
    - Unlikely
level: high

Original YAML and metadata in JSON

False positives

• Unlikely

Source references

• https://github.com/Neo23x0/Raccine/blob/20a569fa21625086433dcce8bb2765d0ea08dcb6/yara/mal_revil.yar
• https://thedfirreport.com/2022/05/09/seo-poisoning-a-gootloader-story/