GNU InetUtils telnetd Remote Authentication Bypass Vulnerability

Critical Vulnerability Information Vulnerability Overview Vulnerability Name: GNU InetUtils Security Advisory: remote authentication bypass in telnet Vulnerability Type: Remote Authentication Bypass Severity: High Vulnerability Details Affected Versions: GNU InetUtils versions 1.9.3 and later, up to version 2.7 Vulnerability Description: - The telnetd server passes the value of the environment variable received from the client as the last argument to . - If the client provides a carefully crafted environment variable value, such as the string "-f root", and uses the or parameter, the client will automatically log in as root, bypassing normal authentication procedures. Impact The telnetd server does not sanitize the environment variable, passing it directly to the program, which uses the parameter to bypass normal authentication. Mitigation Measures Recommended Actions: - Do not run the telnetd service. - Restrict network access to the telnet port, allowing only trusted clients. - Apply patches or upgrade to the latest version containing the fix. Workarounds: - Disable the telnetd server, or use a custom tool that does not support the parameter. Vulnerability History Introduced: In a commit on March 19, 2015. Disclosure Date: January 20, 2026. Related Links Commit Record: Commit introducing the vulnerability Mailing List Discussions: - Discussion 1 - Discussion 2 Reporters and Contributors Discoverer: Kyu Neushwaistein, reported on January 19, 2026. Initial Patch: Paul Eggert, January 20, 2026. Patch Improvements: Simon Josefsson. Advisory Drafted By: Simon Josefsson, January 20, 2026. Code Snippet Examples and Patches Example code demonstrates how to exploit the vulnerability by crafting malicious requests. Patch recommendations include sanitizing all environment variables; specific patch links are available in the original document.

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GNU InetUtils telnetd Remote Authentication Bypass Vulnerability