Potential Base64 Decoded From Images — Sigma Rule

Rule metadata and linked tags

Author: Joseliyo Sanchez, @Joseliyo_Jstnk. Source status: test; severity: high. Source dates: 2023-12-20 / not supplied.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 688aa4542fa6127e73cb4effded3baf6e36fb9d572db14884a6ae73436b76615

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 Base64 Decoded From Images
id: 09a910bf-f71f-4737-9c40-88880ba5913d
status: test
description: |
    Detects the use of tail to extract bytes at an offset from an image and then decode the base64 value to create a new file with the decoded content. The detected execution is a bash one-liner.
references:
    - https://www.virustotal.com/gui/file/16bafdf741e7a13137c489f3c8db1334f171c7cb13b62617d691b0a64783cc48/behavior
    - https://www.virustotal.com/gui/file/483fafc64a2b84197e1ef6a3f51e443f84dc5742602e08b9e8ec6ad690b34ed0/behavior
author: Joseliyo Sanchez, @Joseliyo_Jstnk
date: 2023-12-20
tags:
    - attack.stealth
    - attack.t1140
logsource:
    product: macos
    category: process_creation
detection:
    # Example:  /bin/bash sh -c tail -c +21453 '/Volumes/Installer/Installer.app/Contents/Resources/workout-logo.jpeg' | base64 --decode > /tmp/54A0A2CD-FAD1-4D4D-AAF5-5266F6344ABE.zip
    # VT Query: 'behavior_processes:"tail" (behavior_processes:"jpeg" or behavior_processes:"jpg" or behavior_processes:"png" or behavior_processes:"gif") behavior_processes:"base64" behavior_processes:"--decode >" and tag:dmg'
    selection_image:
        Image|endswith: '/bash'
    selection_view:
        CommandLine|contains|all:
            - 'tail'
            - '-c'
    selection_b64:
        CommandLine|contains|all:
            - 'base64'
            - '-d' # Also covers "--decode"
            - '>'
    selection_files:
        CommandLine|contains:
            - '.avif'
            - '.gif'
            - '.jfif'
            - '.jpeg'
            - '.jpg'
            - '.pjp'
            - '.pjpeg'
            - '.png'
            - '.svg'
            - '.webp'
    condition: all of selection_*
falsepositives:
    - Unknown
level: high

Original YAML and metadata in JSON

False positives

• Unknown

Source references

• https://www.virustotal.com/gui/file/16bafdf741e7a13137c489f3c8db1334f171c7cb13b62617d691b0a64783cc48/behavior
• https://www.virustotal.com/gui/file/483fafc64a2b84197e1ef6a3f51e443f84dc5742602e08b9e8ec6ad690b34ed0/behavior