Telnet Authentication Bypass via User Environment Variable

  1[metadata]
  2creation_date = "2026/01/26"
  3integration = ["auditd_manager"]
  4maturity = "production"
  5updated_date = "2026/01/26"
  6
  7[rule]
  8author = ["Elastic"]
  9description = """
 10Identifies potential exploitation of a Telnet remote authentication bypass vulnerability (CVE-2026-24061) in GNU Inetutils
 11telnetd. The vulnerability allows unauthenticated access by supplying a crafted `-f <username>` value via the `USER` environment
 12variable, resulting in a login process spawned with elevated privileges.
 13"""
 14from = "now-9m"
 15index = ["auditbeat-*", "logs-auditd_manager.auditd-*"]
 16language = "eql"
 17license = "Elastic License v2"
 18name = "Telnet Authentication Bypass via User Environment Variable"
 19note = """## Triage and analysis
 20
 21### Investigating Telnet Authentication Bypass via User Environment Variable
 22
 23CVE-2026-24061 is a critical authentication bypass vulnerability affecting `telnetd` in GNU Inetutils. By supplying a
 24crafted `-f root` value through the USER environment variable, a remote attacker can bypass authentication and gain
 25unauthorized root-level access. This exploit results in the `login` process being executed with attacker-controlled
 26arguments, typically spawned by `telnetd` or via `xinetd`.
 27
 28This rule detects suspicious `login` executions associated with Telnet services that include the `-f` flag, which
 29forces authentication as a specified user and is indicative of exploitation attempts.
 30
 31#### Possible investigation steps
 32
 33- Investigate the process execution chain (parent process tree) for the suspicious `login` process.
 34  - Confirm whether `login` was spawned by `telnetd` or indirectly via `xinetd`.
 35  - Review the command-line arguments passed to `login`, paying special attention to the presence of `-f` and any
 36    attempts to authenticate as `root` or other privileged users.
 37- Validate whether the Telnet service is expected to be running on the affected host.
 38  - Telnet is deprecated and should rarely be exposed or enabled in modern environments.
 39- Investigate post-authentication activity originating from the compromised session.
 40  - Look for command execution, file modifications, privilege escalation attempts, or persistence mechanisms.
 41  - Review network connections initiated after the suspicious login event.
 42- Check for additional alerts or suspicious activity on the same host within the past 48 hours.
 43- Determine whether the system is running a vulnerable version of GNU Inetutils telnetd.
 44
 45### False positive analysis
 46
 47- Legitimate use of the `-f` flag with `login` is extremely rare and typically restricted to trusted, local workflows.
 48- False positives may occur in highly customized or legacy environments where Telnet is still in use.
 49- Any benign occurrences should be carefully validated and documented before adding exceptions.
 50
 51### Related Rules
 52
 53- Potential Telnet Authentication Bypass (CVE-2026-24061) - "ab7795cc-0e0b-4f9d-a934-1f17a58f869a"
 54
 55### Response and remediation
 56
 57- Immediately isolate the affected host to prevent further unauthorized access or lateral movement.
 58- Terminate suspicious Telnet sessions and collect volatile forensic data where possible.
 59- Investigate for signs of credential access, persistence, or follow-on exploitation.
 60- Patch or upgrade GNU Inetutils to a version that addresses CVE-2026-24061.
 61- Disable the Telnet service entirely if it is not explicitly required.
 62- Enforce the use of secure alternatives such as SSH for remote administration.
 63- Rotate credentials for any accounts that may have been exposed or accessed.
 64- Perform a full system integrity review and antimalware scan.
 65- Update hardening, monitoring, and logging policies to improve detection of legacy remote access abuse.
 66"""
 67references = [
 68    "https://www.safebreach.com/blog/safebreach-labs-root-cause-analysis-and-poc-exploit-for-cve-2026-24061/",
 69    "https://security-tracker.debian.org/tracker/CVE-2026-24061"
 70]
 71risk_score = 99
 72rule_id = "eb3150eb-e9fb-4a64-a0fc-aa66cdd35632"
 73severity = "critical"
 74tags = [
 75    "Domain: Endpoint",
 76    "OS: Linux",
 77    "Use Case: Threat Detection",
 78    "Tactic: Initial Access",
 79    "Tactic: Lateral Movement",
 80    "Resources: Investigation Guide",
 81    "Use Case: Vulnerability",
 82    "Data Source: Auditd Manager"
 83]
 84timestamp_override = "event.ingested"
 85type = "eql"
 86
 87query = '''
 88sequence by host.id with maxspan=1s
 89 [process where host.os.type == "linux" and event.type == "start" and event.action in ("process_started", "executed") and process.name == "telnetd"] by process.pid
 90 [process where host.os.type == "linux" and event.type == "start" and event.action in ("process_started", "executed") and process.name == "login" and process.args : "-*f*"] by process.parent.pid
 91'''
 92
 93
 94[[rule.threat]]
 95framework = "MITRE ATT&CK"
 96[[rule.threat.technique]]
 97id = "T1190"
 98name = "Exploit Public-Facing Application"
 99reference = "https://attack.mitre.org/techniques/T1190/"
100
101
102[rule.threat.tactic]
103id = "TA0001"
104name = "Initial Access"
105reference = "https://attack.mitre.org/tactics/TA0001/"
106[[rule.threat]]
107framework = "MITRE ATT&CK"
108[[rule.threat.technique]]
109id = "T1210"
110name = "Exploitation of Remote Services"
111reference = "https://attack.mitre.org/techniques/T1210/"
112
113
114[rule.threat.tactic]
115id = "TA0008"
116name = "Lateral Movement"
117reference = "https://attack.mitre.org/tactics/TA0008/"