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Telnet Authentication Bypass via User Environment Variable

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Telnet Authentication Bypass via User Environment Variable

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Identifies potential exploitation of a Telnet remote authentication bypass vulnerability (CVE-2026-24061) in GNU Inetutils telnetd. The vulnerability allows unauthenticated access by supplying a crafted -f <username> value via the USER environment variable, resulting in a login process spawned with elevated privileges.

Rule type: eql

Rule indices:

  • auditbeat-*
  • logs-auditd_manager.auditd-*

Severity: critical

Risk score: 99

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Domain: Endpoint
  • OS: Linux
  • Use Case: Threat Detection
  • Tactic: Initial Access
  • Tactic: Lateral Movement
  • Resources: Investigation Guide
  • Use Case: Vulnerability
  • Data Source: Auditd Manager

Version: 1

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Investigating Telnet Authentication Bypass via User Environment Variable

CVE-2026-24061 is a critical authentication bypass vulnerability affecting telnetd in GNU Inetutils. By supplying a crafted -f root value through the USER environment variable, a remote attacker can bypass authentication and gain unauthorized root-level access. This exploit results in the login process being executed with attacker-controlled arguments, typically spawned by telnetd or via xinetd.

This rule detects suspicious login executions associated with Telnet services that include the -f flag, which forces authentication as a specified user and is indicative of exploitation attempts.

Possible investigation steps

  • Investigate the process execution chain (parent process tree) for the suspicious login process.
  • Confirm whether login was spawned by telnetd or indirectly via xinetd.
  • Review the command-line arguments passed to login, paying special attention to the presence of -f and any attempts to authenticate as root or other privileged users.
  • Validate whether the Telnet service is expected to be running on the affected host.
  • Telnet is deprecated and should rarely be exposed or enabled in modern environments.
  • Investigate post-authentication activity originating from the compromised session.
  • Look for command execution, file modifications, privilege escalation attempts, or persistence mechanisms.
  • Review network connections initiated after the suspicious login event.
  • Check for additional alerts or suspicious activity on the same host within the past 48 hours.
  • Determine whether the system is running a vulnerable version of GNU Inetutils telnetd.

False positive analysis

  • Legitimate use of the -f flag with login is extremely rare and typically restricted to trusted, local workflows.
  • False positives may occur in highly customized or legacy environments where Telnet is still in use.
  • Any benign occurrences should be carefully validated and documented before adding exceptions.

Related Rules

  • Potential Telnet Authentication Bypass (CVE-2026-24061) - "ab7795cc-0e0b-4f9d-a934-1f17a58f869a"

Response and remediation

  • Immediately isolate the affected host to prevent further unauthorized access or lateral movement.
  • Terminate suspicious Telnet sessions and collect volatile forensic data where possible.
  • Investigate for signs of credential access, persistence, or follow-on exploitation.
  • Patch or upgrade GNU Inetutils to a version that addresses CVE-2026-24061.
  • Disable the Telnet service entirely if it is not explicitly required.
  • Enforce the use of secure alternatives such as SSH for remote administration.
  • Rotate credentials for any accounts that may have been exposed or accessed.
  • Perform a full system integrity review and antimalware scan.
  • Update hardening, monitoring, and logging policies to improve detection of legacy remote access abuse.

Rule query

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sequence by host.id with maxspan=1s
 [process where host.os.type == "linux" and event.type == "start" and event.action in ("process_started", "executed") and process.name == "telnetd"] by process.pid
 [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

Framework: MITRE ATT&CKTM

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