splunk-security_content/detections/endpoint/linux_decode_base64_to_shell.yml

name: Linux Decode Base64 to Shell
id: 637b603e-1799-40fd-bf87-47ecbd551b66
version: 2
date: '2024-05-22'
author: Michael Haag, Splunk
status: production
type: TTP
description: |-
  The following analytic detects the decoding of base64-encoded data and its execution in a Linux shell. It leverages the Endpoint.Processes data model to search for commands like "base64 -d" and "base64 --decode" combined with Linux shell execution. This activity is significant because base64 encoding is often used to obfuscate malicious commands or payloads, indicating potential malicious activity. If confirmed malicious, this behavior could allow an attacker to execute unauthorized commands, gain unauthorized access, exfiltrate data, or perform other harmful actions on the Linux system.
data_source:
- Sysmon for Linux EventID 1
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: The detection is based on data that originates from Endpoint Detection
  and Response (EDR) agents. These agents are designed to provide security-related
  telemetry from the endpoints where the agent is installed. To implement this search,
  you must ingest logs that contain the process GUID, process name, and parent process.
  Additionally, you must ingest complete command-line executions. These logs must
  be processed using the appropriate Splunk Technology Add-ons that are specific to
  the EDR product. The logs must also be mapped to the `Processes` node of the `Endpoint`
  data model. Use the Splunk Common Information Model (CIM) to normalize the field
  names and speed up the data modeling process.
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
  confidence: 50
  impact: 50
  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
  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: 25
  security_domain: endpoint
tests:
- name: True Positive Test
  attack_data:
  - data: 
      https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1027/atomic_red_team/linux-sysmon.log
    source: Syslog:Linux-Sysmon/Operational
    sourcetype: sysmon:linux
    update_timestamp: true