Linux Base64 Encoded Shebang In CLI — Sigma Rule

Rule metadata and linked tags

Author: Nasreddine Bencherchali (Nextron Systems). Source status: test; severity: medium. Source dates: 2022-09-15 / not supplied.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: d15be8f68f508302af3c81e03407aab24c0558ed145005e12ad5a357439a0db3

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: Linux Base64 Encoded Shebang In CLI
id: fe2f9663-41cb-47e2-b954-8a228f3b9dff
status: test
description: Detects the presence of a base64 version of the shebang in the commandline, which could indicate a malicious payload about to be decoded
references:
    - https://www.trendmicro.com/pl_pl/research/20/i/the-evolution-of-malicious-shell-scripts.html
    - https://github.com/carlospolop/PEASS-ng/tree/master/linPEAS
author: Nasreddine Bencherchali (Nextron Systems)
date: 2022-09-15
tags:
    - attack.stealth
    - attack.t1140
logsource:
    product: linux
    category: process_creation
detection:
    selection:
        CommandLine|contains:
            - "IyEvYmluL2Jhc2" # Note: #!/bin/bash"
            - "IyEvYmluL2Rhc2" # Note: #!/bin/dash"
            - "IyEvYmluL3pza" # Note: #!/bin/zsh"
            - "IyEvYmluL2Zpc2" # Note: #!/bin/fish
            - "IyEvYmluL3No" # Note: # !/bin/sh"
    condition: selection
falsepositives:
    - Legitimate administration activities
level: medium

Original YAML and metadata in JSON

False positives

• Legitimate administration activities

Source references

• https://www.trendmicro.com/pl_pl/research/20/i/the-evolution-of-malicious-shell-scripts.html
• https://github.com/carlospolop/PEASS-ng/tree/master/linPEAS