name: CertUtil With Decode Argument
id: bfe94226-8c10-11eb-a4b3-acde48001122
version: 2
date: '2021-03-23'
author: Michael Haag, Splunk
type: TTP
datamodel:
- Endpoint
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.
search: '| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time)
  as lastTime from datamodel=Endpoint.Processes where `process_certutil` Processes.process=*decode*
  by Processes.dest Processes.user Processes.parent_process Processes.process_name
  Processes.process Processes.process_id Processes.parent_process_id | `drop_dm_object_name(Processes)`
  | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `certutil_with_decode_argument_filter`'
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
  confidence: 80
  context:
  - Source:Endpoint
  - Stage:Execution
  dataset:
  - https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1140/atomic_red_team/windows-sysmon.log
  impact: 50
  kill_chain_phases:
  - Exploitation
  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
  required_fields:
  - _time
  - Processes.dest
  - Processes.user
  - Processes.parent_process_name
  - Processes.parent_process
  - Processes.original_file_name
  - Processes.process_name
  - Processes.process
  - Processes.process_id
  - Processes.parent_process_path
  - Processes.process_path
  - Processes.parent_process_id
  risk_score: 40
  security_domain: endpoint
  supported_tas:
  - Splunk_TA_microsoft_sysmon
  asset_type: Endpoint