splunk-security_content/detections/endpoint/linux_ssh_authorized_keys_modification.yml

name: Linux SSH Authorized Keys Modification
id: f5ab595e-28e5-4327-8077-5008ba97c850
version: 4
date: '2024-09-30'
author: Michael Haag, Splunk
status: production
type: Anomaly
description: The following analytic detects the modification of SSH Authorized Keys on Linux systems. It leverages process execution data from Endpoint Detection and Response (EDR) agents, specifically monitoring commands like "bash" and "cat" interacting with "authorized_keys" files. This activity is significant as adversaries often modify SSH Authorized Keys to establish persistent access to compromised endpoints. If confirmed malicious, this behavior could allow attackers to maintain unauthorized access, bypassing traditional authentication mechanisms and potentially leading to further exploitation or data exfiltration.
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_name IN ("bash","cat") Processes.process IN ("*/authorized_keys*") 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_ssh_authorized_keys_modification_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: Filtering will be required as system administrators will add and remove. One way to filter query is to add "echo".
references:
- https://redcanary.com/blog/lateral-movement-with-secure-shell/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1098.004/T1098.004.md
drilldown_searches:
- name: View the detection results for - "$user$" and "$dest$"
  search: '%original_detection_search% | search  user = "$user$" dest = "$dest$"'
  earliest_offset: $info_min_time$
  latest_offset: $info_max_time$
- name: View risk events for the last 7 days for - "$user$" and "$dest$"
  search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$", "$dest$") starthoursago=168  | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
  earliest_offset: $info_min_time$
  latest_offset: $info_max_time$
tags:
  analytic_story:
  - Linux Living Off The Land
  asset_type: Endpoint
  confidence: 50
  impact: 30
  message: An instance of $parent_process_name$ spawning $process_name$ was identified on endpoint $dest$ by user $user$ modifying SSH Authorized Keys.
  mitre_attack_id:
  - T1098.004
  observable:
  - name: user
    type: User
    role:
    - Victim
  - name: dest
    type: Hostname
    role:
    - Victim
  - name: parent_process_name
    type: Process
    role:
    - Attacker
  - name: process_name
    type: Process
    role:
    - Attacker
  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: 15
  security_domain: endpoint
tests:
- name: True Positive Test
  attack_data:
  - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1098.004/ssh_authorized_keys/authkey_linux-sysmon.log
    source: Syslog:Linux-Sysmon/Operational
    sourcetype: sysmon:linux
    update_timestamp: true