Summary
CVE-2026-24061 is a critical authentication bypass vulnerability in GNU InetUtils telnetd. It allows unauthenticated remote attackers to gain immediate root access by injecting crafted values into environment variables handled during the Telnet session setup. The issue stems from unsafe handling of client-controlled input that is passed directly into the system authentication process.
Overview
The vulnerability affects GNU InetUtils telnetd versions from 1.9.3 through 2.7. It has a CVSS score of 9.8 and is exploitable over the network without authentication. The flaw results in full remote code execution as root.
Telnet remains present in legacy systems, embedded devices, and operational technology environments. Many such systems are still exposed on port 23, increasing the practical risk of exploitation in real-world deployments.
Technical Analysis
How Authentication Works in telnetd
The telnet daemon does not handle authentication internally. Instead, it delegates authentication to the system binary:
/usr/bin/login
When a client connects using the **Telnet protocol, telnetd constructs a command that passes connection-related parameters (such as hostname and username) to /usr/bin/login.
Root Cause
The vulnerability originates from how telnetd processes the USER environment variable. During session negotiation, the Telnet protocol allows clients to define environment variables using the NEW_ENVIRON option. telnetd reads this value and inserts it into the login command arguments.
The critical issue is that this value is not validated or sanitized before being used.
Relevant code behavior:
case 'U':
return getenv("USER") ? xstrdup(getenv("USER")) : xstrdup("");This directly passes attacker-controlled input into a privileged command execution context.
Exploitation Mechanism
An attacker can supply a malicious value for the USER variable during the Telnet handshake.
Example:
USER = "-f root"
This modifies the login command to:
/usr/bin/login -h <hostname> "-f root"The -f flag instructs the login program to skip authentication and directly log in as the specified user. Since the injected value includes root, the attacker obtains a root shell without providing credentials.
This is a classic case of argument injection, where user input is interpreted as command-line options rather than data.
Impact
Successful exploitation results in immediate root-level access. This allows full control over the affected system, including executing commands, accessing sensitive data, modifying configurations, and moving laterally within a network.
The vulnerability is particularly critical in environments where telnetd is still used, such as embedded systems and legacy infrastructure, where patching may be infrequent.
Patch and Fix
The issue was addressed by introducing input sanitization. The patch adds a function that filters unsafe values, specifically:
- Rejects inputs starting with - (to prevent flag injection)
- Blocks shell metacharacters
Example of patched logic:
static char *sanitize (const char *u)
{
if (u && *u != '-' && !u[strcspn (u, "\t\n !\"#$&'()*;<=>?[\\^`{|}~")])
return u;
else
return "";
}All relevant inputs, including the USER variable, are now passed through this sanitization layer before being used.
Mitigation
Systems should be updated to GNU InetUtils version 2.7-2 or later.
If patching is not immediately possible, disabling telnetd is recommended. Blocking port 23 at the network level can reduce exposure, but it should not be considered a complete solution.
In environments that still rely on Telnet, access should be restricted through network segmentation or controlled access mechanisms such as VPNs.
Long-Term Recommendation
Telnet should be phased out in favor of secure alternatives such as OpenSSH. Unlike Telnet, SSH provides encrypted communication and avoids reliance on insecure protocol-level features such as client-controlled environment variables.
Conclusion
CVE-2026-24061 is a straightforward but high-impact vulnerability caused by improper handling of user-controlled input. The lack of validation allowed attackers to inject command-line arguments into a privileged authentication process, resulting in complete authentication bypass.
The vulnerability highlights a recurring issue in system-level software: trusting externally supplied data in security-critical execution paths.
References
- NIST NVD entry (CVE-2026-24061)
https://nvd.nist.gov/vuln/detail/CVE-2026-24061 - Fix commit (sanitization patch)
https://codeberg.org/inetutils/inetutils/commit/ccba9f748aa8d50a38d7748e2e60362edd6a32cc
