name: CertUtil With Decode Argument
id: bfe94226-8c10-11eb-a4b3-acde48001122
version: 2
date: '2021-03-23'
author: Michael Haag, Splunk
status: production
type: TTP
description: CertUtil.exe may be used to `encode` and `decode` a file, including PE
  and script code. Encoding will convert a file to base64 with `-----BEGIN CERTIFICATE-----`
  and `-----END CERTIFICATE-----` tags. Malicious usage will include decoding a encoded
  file that was downloaded. Once decoded, it will be loaded by a parallel process.
  Note that there are two additional command switches that may be used - `encodehex`
  and `decodehex`. Similarly, the file will be encoded in HEX and later decoded for
  further execution. During triage, identify the source of the file being decoded.
  Review its contents or execution behavior for further analysis.
data_source:
- Sysmon Event ID 1
search:
  selection1:
    OriginalFileName: CertUtil.exe
  selection2:
    Image|endswith: certutil.exe
  selection3:
    CommandLine: '*decode*'
  condition: (selection1 or selection2) and selection3
how_to_implement: To successfully implement this search you need to be ingesting information
  on process that include the name of the process responsible for the changes from
  your endpoints into the `Endpoint` datamodel in the `Processes` node. In addition,
  confirm the latest CIM App 4.20 or higher is installed and the latest TA for the
  endpoint product.
known_false_positives: Typically seen used to `encode` files, but it is possible to
  see legitimate use of `decode`. Filter based on parent-child relationship, file
  paths, endpoint or user.
references:
- https://attack.mitre.org/techniques/T1140/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1140/T1140.md
- https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/certutil
- https://www.bleepingcomputer.com/news/security/certutilexe-could-allow-attackers-to-download-malware-while-bypassing-av/
tags:
  analytic_story:
  - Deobfuscate-Decode Files or Information
  - Living Off The Land
  - Forest Blizzard
  asset_type: Endpoint
  confidence: 80
  impact: 50
  message: An instance of $parent_process_name$ spawning $process_name$ was identified
    on endpoint $dest$ by user $user$ attempting to decode a file.
  mitre_attack_id:
  - T1140
  observable:
  - name: user
    type: User
    role:
    - Victim
  - name: dest
    type: Hostname
    role:
    - Victim
  - name: parent_process_name
    type: Process
    role:
    - Parent Process
  - name: process_name
    type: Process
    role:
    - Child Process
  product:
  - Splunk Enterprise
  - Splunk Enterprise Security
  - Splunk Cloud
  risk_score: 40
  security_domain: endpoint
tests:
- name: True Positive Test
  attack_data:
  - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1140/atomic_red_team/windows-sysmon.log
    source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
    sourcetype: xmlwineventlog