Critical Authentication Bypass Flaw in GNU InetUtils Telnetd Grants Root Access Without Credentials
A high-severity vulnerability in GNU InetUtils’ telnetd server (versions 1.9.3 through 2.7) allows unauthenticated remote attackers to bypass authentication and gain root access by exploiting improper input sanitization. The flaw, introduced in a 2015 commit (fa3245ac), stems from the USER environment variable being passed unsanitized to the login utility, which interprets the -f flag as an authentication bypass.
Technical Details
The vulnerability resides in telnetd/utility.c, where the _var_short_name() function fails to validate the %U parameter (representing the USER variable) in the login command template:
PATH_LOGIN -p -h %h %?u{-f %u}{%U}.
An attacker can inject -f root via the USER variable, tricking the login program into granting root privileges without credentials.
Exploitation & Impact
A proof-of-concept exploit requires only a single command:
USER='-f root' telnet -a localhost
This immediately spawns a root shell without password authentication, as demonstrated on Trisquel GNU/Linux 11. The flaw affects all versions since v1.9.3 (May 2015) and remains unpatched in v2.7 unless mitigated.
Recommended Actions
The GNU InetUtils team advises disabling telnetd entirely, as modern systems should use SSH for secure remote access. Patches (commits fd702c02 and ccba9f748) introduce variable sanitization to block similar attacks. Network administrators are urged to restrict telnet port access and migrate to SSH-based solutions. Custom login tools that reject the -f parameter are also suggested as a workaround.
Source: https://gbhackers.com/gnu-inetutils-vulnerability-exploited/
GNU Project cybersecurity rating report: https://www.rankiteo.com/company/gnu-project
"id": "GNU1769023602",
"linkid": "gnu-project",
"type": "Vulnerability",
"date": "5/2015",
"severity": "100",
"impact": "5",
"explanation": "Attack threatening the organization's existence"{'affected_entities': [{'customers_affected': 'Users of GNU InetUtils telnetd '
'(versions 1.9.3 through 2.7)',
'industry': 'Open Source Software',
'name': 'GNU InetUtils',
'type': 'Software Project'}],
'attack_vector': 'Remote Exploitation',
'description': 'A high-severity vulnerability in GNU InetUtils’ telnetd '
'server (versions 1.9.3 through 2.7) allows unauthenticated '
'remote attackers to bypass authentication and gain root '
'access by exploiting improper input sanitization. The flaw '
'stems from the USER environment variable being passed '
'unsanitized to the login utility, which interprets the -f '
'flag as an authentication bypass.',
'impact': {'operational_impact': 'Unauthorized root access to affected '
'systems',
'systems_affected': 'GNU InetUtils telnetd (versions 1.9.3 through '
'2.7)'},
'lessons_learned': 'Modern systems should use SSH instead of telnet for '
'secure remote access; input sanitization is critical to '
'prevent authentication bypass vulnerabilities.',
'post_incident_analysis': {'corrective_actions': 'Sanitize USER variable in '
'login command template; '
'deprecate telnetd in favor '
'of SSH',
'root_causes': 'Improper input sanitization in '
'telnetd/utility.c, allowing USER '
'environment variable injection to '
'bypass authentication'},
'recommendations': ['Disable telnetd entirely and migrate to SSH-based '
'solutions',
'Apply patches (commits fd702c02 and ccba9f748) to '
'sanitize USER variable',
'Restrict telnet port access at the network level',
'Use custom login tools that reject the -f parameter as a '
'workaround'],
'references': [{'source': 'GNU InetUtils Advisory'}],
'response': {'containment_measures': 'Disable telnetd entirely; restrict '
'telnet port access',
'remediation_measures': 'Apply patches (commits fd702c02 and '
'ccba9f748) to sanitize USER variable; '
'migrate to SSH-based solutions'},
'title': 'Critical Authentication Bypass Flaw in GNU InetUtils Telnetd Grants '
'Root Access Without Credentials',
'type': 'Authentication Bypass',
'vulnerability_exploited': 'Improper input sanitization in GNU InetUtils '
'telnetd (USER environment variable handling)'}