name: Linux Decode Base64 to Shell
id: 637b603e-1799-40fd-bf87-47ecbd551b66
version: 1
date: '2022-07-27'
author: Michael Haag, Splunk
type: TTP
datamodel:
- Endpoint
description: The following analytic identifies base64 being decoded and passed to a Linux shell.
search: '| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time)
  as lastTime from datamodel=Endpoint.Processes where Processes.process IN ("*base64 -d*","*base64 --decode*") AND Processes.process="*|*" `linux_shells`
   by Processes.dest Processes.user Processes.parent_process_name 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)`
  | `linux_decode_base64_to_shell_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: False positives may be present based on legitimate software being utilized. Filter as needed.
references:
  - https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1027/T1027.md#atomic-test-1---decode-base64-data-into-script
  - https://redcanary.com/blog/lateral-movement-with-secure-shell/ 
  - https://linux.die.net/man/1/base64
tags:
  analytic_story:
  - Linux Living Off The Land
  asset_type: Endpoint
  cis20:
  - CIS 3
  - CIS 5
  - CIS 16
  confidence: 50
  context:
  - Source:Endpoint
  - Stage:Defense Evasion
  dataset:
  - https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1027/atomic_red_team/linux-sysmon.log
  impact: 50
  kill_chain_phases:
  - Delivery
  - Exploitation
  message: An instance of $parent_process_name$ spawning $process_name$ was identified on endpoint $dest$ by user $user$ decoding base64 and passing it to a shell.
  mitre_attack_id:
  - T1027
  - T1059.004
  nist:
  - DE.CM
  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 #parent process name
  - Processes.parent_process #parent cmdline
  - Processes.original_file_name
  - Processes.process_name #process name
  - Processes.process #process cmdline
  - Processes.process_id
  - Processes.parent_process_path
  - Processes.process_path
  - Processes.parent_process_id
  risk_score: 25
  security_domain: endpoint