---
title: Telnet Authentication Bypass via User Environment Variable
description: Identifies potential exploitation of a Telnet remote authentication bypass vulnerability (CVE-2026-24061) in GNU Inetutils telnetd. The vulnerability...
url: https://www.elastic.co/docs/reference/security/prebuilt-rules/rules/linux/initial_access_telnet_auth_bypass_envar_auditd
products:
  - Elastic Security
---

# Telnet Authentication Bypass via User Environment Variable
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-*

**Rule Severity**: critical
**Risk Score**: 99
**Runs every**: 
**Searches indices from**: `now-9m`
**Maximum alerts per execution**: 100
**References**:
- [[https://www.safebreach.com/blog/safebreach-labs-root-cause-analysis-and-poc-exploit-for-cve-2026-24061/](https://www.safebreach.com/blog/safebreach-labs-root-cause-analysis-and-poc-exploit-for-cve-2026-24061/)](https://www.safebreach.com/blog/safebreach-labs-root-cause-analysis-and-poc-exploit-for-cve-2026-24061/)
- [[https://security-tracker.debian.org/tracker/CVE-2026-24061](https://security-tracker.debian.org/tracker/CVE-2026-24061)](https://security-tracker.debian.org/tracker/CVE-2026-24061)

**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
- Noise: Low
- Performance: Normal
- Profile: Recommended
- Threat: Vulnerability Exploit
- Rule Type: Event Correlation (EQL)
- Platform: Linux
- Vuln: CVE-2026-24061

**Version**: 4
**Rule authors**:
- Elastic

**Rule license**: Elastic License v2

## Investigation guide


## 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

```eql
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 in ("telnetd", "xinetd")] 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&CK
- Tactic:
  - Name: Initial Access
  - Id: TA0001
  - Reference URL: [[https://attack.mitre.org/tactics/TA0001/](https://attack.mitre.org/tactics/TA0001/)](https://attack.mitre.org/tactics/TA0001/)
- Technique:
  - Name: Exploit Public-Facing Application
  - Id: T1190
  - Reference URL: [[https://attack.mitre.org/techniques/T1190/](https://attack.mitre.org/techniques/T1190/)](https://attack.mitre.org/techniques/T1190/)

**Framework:** MITRE ATT&CK
- Tactic:
  - Name: Lateral Movement
  - Id: TA0008
  - Reference URL: [[https://attack.mitre.org/tactics/TA0008/](https://attack.mitre.org/tactics/TA0008/)](https://attack.mitre.org/tactics/TA0008/)
- Technique:
  - Name: Exploitation of Remote Services
  - Id: T1210
  - Reference URL: [[https://attack.mitre.org/techniques/T1210/](https://attack.mitre.org/techniques/T1210/)](https://attack.mitre.org/techniques/T1210/)

**Framework:** MITRE ATT&CK
- Tactic:
  - Name: Privilege Escalation
  - Id: TA0004
  - Reference URL: [[https://attack.mitre.org/tactics/TA0004/](https://attack.mitre.org/tactics/TA0004/)](https://attack.mitre.org/tactics/TA0004/)
- Technique:
  - Name: Exploitation for Privilege Escalation
  - Id: T1068
  - Reference URL: [[https://attack.mitre.org/techniques/T1068/](https://attack.mitre.org/techniques/T1068/)](https://attack.mitre.org/techniques/T1068/)