Suspicious File Encoded To Base64 Via Certutil.EXE — Sigma Rule

Rule metadata and linked tags

Author: Nasreddine Bencherchali (Nextron Systems). Source status: test; severity: high. Source dates: 2023-05-15 / 2024-03-05.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 101a627391a63cd14ccb58ef7374dc59df7d87a8facab601e7a9a24726fe64c1

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 File Encoded To Base64 Via Certutil.EXE
id: ea0cdc3e-2239-4f26-a947-4e8f8224e464
related:
    - id: e62a9f0c-ca1e-46b2-85d5-a6da77f86d1a
      type: derived
status: test
description: Detects the execution of certutil with the "encode" flag to encode a file to base64 where the extensions of the file is suspicious
references:
    - https://www.virustotal.com/gui/file/35c22725a92d5cb1016b09421c0a6cdbfd860fd4778b3313669b057d4a131cb7/behavior
    - https://www.virustotal.com/gui/file/427616528b7dbc4a6057ac89eb174a3a90f7abcf3f34e5a359b7a910d82f7a72/behavior
    - https://www.virustotal.com/gui/file/34de4c8beded481a4084a1fd77855c3e977e8ac643e5c5842d0f15f7f9b9086f/behavior
    - https://www.virustotal.com/gui/file/4abe1395a09fda06d897a9c4eb247278c1b6cddda5d126ce5b3f4f499e3b8fa2/behavior
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-05-15
modified: 2024-03-05
tags:
    - attack.stealth
    - attack.t1027
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith: '\certutil.exe'
        - OriginalFileName: 'CertUtil.exe'
    selection_cli:
        CommandLine|contains|windash: '-encode'
    selection_extension:
        CommandLine|contains:
            - '.acl'
            - '.bat'
            - '.doc'
            - '.gif'
            - '.jpeg'
            - '.jpg'
            - '.mp3'
            - '.pdf'
            - '.png'
            - '.ppt'
            - '.tmp'
            - '.xls'
            - '.xml'
    condition: all of selection_*
falsepositives:
    - Unknown
level: high
regression_tests_path: regression_data/rules/windows/process_creation/proc_creation_win_certutil_encode_susp_extensions/info.yml

Original YAML and metadata in JSON

False positives

• Unknown

Source references

• https://www.virustotal.com/gui/file/35c22725a92d5cb1016b09421c0a6cdbfd860fd4778b3313669b057d4a131cb7/behavior
• https://www.virustotal.com/gui/file/427616528b7dbc4a6057ac89eb174a3a90f7abcf3f34e5a359b7a910d82f7a72/behavior
• https://www.virustotal.com/gui/file/34de4c8beded481a4084a1fd77855c3e977e8ac643e5c5842d0f15f7f9b9086f/behavior
• https://www.virustotal.com/gui/file/4abe1395a09fda06d897a9c4eb247278c1b6cddda5d126ce5b3f4f499e3b8fa2/behavior