Linux SSH X11 Forwarding

  1[metadata]
  2creation_date = "2023/08/23"
  3integration = ["endpoint", "crowdstrike", "sentinel_one_cloud_funnel"]
  4maturity = "production"
  5updated_date = "2026/09/18"
  6[transform]
  7[[transform.osquery]]
  8label = "Osquery - Retrieve Listening Ports"
  9query = "SELECT pid, address, port, socket, protocol, path FROM listening_ports"
 10
 11[[transform.osquery]]
 12label = "Osquery - Retrieve Open Sockets"
 13query = "SELECT pid, family, remote_address, remote_port, socket, state FROM process_open_sockets"
 14
 15[[transform.osquery]]
 16label = "Osquery - Retrieve Information for a Specific User"
 17query = "SELECT * FROM users WHERE username = {{user.name}}"
 18
 19[[transform.osquery]]
 20label = "Osquery - Investigate the Account Authentication Status"
 21query = "SELECT * FROM logged_in_users WHERE user = {{user.name}}"
 22
 23[[transform.osquery]]
 24label = "Osquery - Retrieve Running Processes by User"
 25query = "SELECT pid, username, name FROM processes p JOIN users u ON u.uid = p.uid ORDER BY username"
 26
 27[[transform.osquery]]
 28label = "Osquery - Retrieve Process Info"
 29query = "SELECT name, cmdline, parent, path, uid FROM processes"
 30
 31
 32[rule]
 33author = ["Elastic"]
 34description = """
 35This rule monitors for X11 forwarding via SSH. X11 forwarding is a feature that allows users to run graphical
 36applications on a remote server and display the application's graphical user interface on their local machine. Attackers
 37can abuse X11 forwarding for tunneling their GUI-based tools, pivot through compromised systems, and create covert
 38communication channels, enabling lateral movement and facilitating remote control of systems within a network.
 39"""
 40from = "now-9m"
 41index = [
 42    "endgame-*",
 43    "logs-crowdstrike.fdr*",
 44    "logs-endpoint.events.process*",
 45    "logs-sentinel_one_cloud_funnel.*",
 46]
 47language = "eql"
 48license = "Elastic License v2"
 49name = "Linux SSH X11 Forwarding"
 50note = """## Triage and analysis
 51
 52### Investigating Linux SSH X11 Forwarding
 53
 54Attackers can leverage SSH X11 forwarding to capture a user's graphical desktop session and potentially execute unauthorized GUI applications remotely.
 55
 56This rule looks for the execution of SSH in conjunction with command line arguments that are capable of setting up X11 forwarding.
 57
 58> **Note**:
 59> This investigation guide uses the [Osquery Markdown Plugin](https://www.elastic.co/guide/en/security/current/invest-guide-run-osquery.html) introduced in Elastic Stack version 8.5.0. Older Elastic Stack versions will display unrendered Markdown in this guide.
 60> This investigation guide uses [placeholder fields](https://www.elastic.co/guide/en/security/current/osquery-placeholder-fields.html) to dynamically pass alert data into Osquery queries. Placeholder fields were introduced in Elastic Stack version 8.7.0. If you're using Elastic Stack version 8.6.0 or earlier, you'll need to manually adjust this investigation guide's queries to ensure they properly run.
 61
 62#### Possible investigation steps
 63
 64- Identify any signs of suspicious network activity or anomalies that may indicate network forwarding activity. This could include unexpected traffic patterns or unusual network behavior.
 65  - Investigate listening ports and open sockets to look for potential protocol tunneling, reverse shells, or data exfiltration.
 66    - $osquery_0
 67    - $osquery_1
 68- Identify the user account that performed the action, analyze it, and check whether it should perform this kind of action.
 69  - $osquery_2
 70- Investigate whether the user is currently logged in and active.
 71  - $osquery_3
 72- Investigate the script execution chain (parent process tree) for unknown processes. Examine their executable files for prevalence and whether they are located in expected locations.
 73  - $osquery_4
 74  - $osquery_5
 75- Investigate other alerts associated with the user/host during the past 48 hours.
 76  - If scripts or executables were dropped, retrieve the files and determine if they are malicious:
 77    - Use a private sandboxed malware analysis system to perform analysis.
 78      - Observe and collect information about the following activities:
 79        - Attempts to contact external domains and addresses.
 80          - Check if the domain is newly registered or unexpected.
 81          - Check the reputation of the domain or IP address.
 82        - File access, modification, and creation activities.
 83
 84### Related rules
 85
 86- Potential Linux Tunneling and/or Port Forwarding - 6ee947e9-de7e-4281-a55d-09289bdf947e
 87
 88### False positive analysis
 89
 90- If this activity is related to new benign software installation activity, consider adding exceptions — preferably with a combination of user and command line conditions.
 91- If this activity is related to a system administrator or developer who uses port tunneling/forwarding for benign purposes, consider adding exceptions for specific user accounts or hosts.
 92- Try to understand the context of the execution by thinking about the user, machine, or business purpose. A small number of endpoints, such as servers with unique software, might appear unusual but satisfy a specific business need.
 93
 94### Response and remediation
 95
 96- Initiate the incident response process based on the outcome of the triage.
 97- Isolate the involved host to prevent further post-compromise behavior.
 98- If the triage identified malware, search the environment for additional compromised hosts.
 99  - Implement temporary network rules, procedures, and segmentation to contain the malware.
100  - Stop suspicious processes.
101  - Immediately block the identified indicators of compromise (IoCs).
102  - Inspect the affected systems for additional malware backdoors, such as reverse shells, reverse proxies, or droppers, that attackers could use to reinfect the system.
103- Remove and block malicious artifacts identified during triage.
104- Investigate credential exposure on systems compromised or used by the attacker to ensure all compromised accounts are identified. Reset passwords for these accounts and other potentially compromised credentials, such as email, business systems, and web services.
105- Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and malware components.
106- Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
107- Leverage the incident response data and logging to improve the mean time to detect (MTTD) and the mean time to respond (MTTR).
108"""
109references = ["https://book.hacktricks.xyz/generic-methodologies-and-resources/tunneling-and-port-forwarding"]
110risk_score = 21
111rule_id = "29f0cf93-d17c-4b12-b4f3-a433800539fa"
112severity = "low"
113tags = [
114    "Domain: Endpoint",
115    "OS: Linux",
116    "Use Case: Threat Detection",
117    "Tactic: Command and Control",
118    "Data Source: Elastic Defend",
119    "Data Source: Elastic Endgame",
120    "Data Source: Crowdstrike",
121    "Data Source: SentinelOne",
122    "Resources: Investigation Guide",
123    "Noise: Medium",
124    "Performance: Normal",
125    "Rule Type: Event Correlation (EQL)",
126    "Platform: Linux",
127    "Resources: Osquery",
128]
129timestamp_override = "event.ingested"
130type = "eql"
131
132query = '''
133process where host.os.type == "linux" and event.type == "start" and event.action in ("exec", "exec_event", "start", "ProcessRollup2") and
134process.name in ("ssh", "sshd") and process.args in ("-X", "-Y") and process.args_count >= 3 and
135process.parent.name in ("bash", "dash", "ash", "sh", "tcsh", "csh", "zsh", "ksh", "fish")
136'''
137
138
139[[rule.threat]]
140framework = "MITRE ATT&CK"
141
142[[rule.threat.technique]]
143id = "T1572"
144name = "Protocol Tunneling"
145reference = "https://attack.mitre.org/techniques/T1572/"
146
147[rule.threat.tactic]
148id = "TA0011"
149name = "Command and Control"
150reference = "https://attack.mitre.org/tactics/TA0011/"
151
152[[rule.threat]]
153framework = "MITRE ATT&CK"
154
155[[rule.threat.technique]]
156id = "T1021"
157name = "Remote Services"
158reference = "https://attack.mitre.org/techniques/T1021/"
159
160[[rule.threat.technique.subtechnique]]
161id = "T1021.004"
162name = "SSH"
163reference = "https://attack.mitre.org/techniques/T1021/004/"
164
165[rule.threat.tactic]
166id = "TA0008"
167name = "Lateral Movement"
168reference = "https://attack.mitre.org/tactics/TA0008/"