Telnet Strikes Back: GNU Inetutils Root Authentication Bypass

It is the year 2026. We have post-quantum cryptography, AI-driven threat hunting, and Zero Trust architectures that verify the pulse of the user before granting access to a spreadsheet. Yet, here we are, staring into the abyss of TCP port 23. Telnet. The cockroach of the internet. The protocol that refuses to die, lingering in legacy routers, embedded IoT devices, and the nightmares of sysadmins everywhere.

Usually, when we talk about Telnet vulnerabilities, we are laughing at cleartext credentials sniffing. But CVE-2026-24061 is different. This isn't about intercepting passwords; it's about skipping the password entirely. It is a bug so stunningly simple, so beautifully archaic, that it feels like it fell out of a Phrack magazine from 1995.

The vulnerability lies in GNU Inetutils—specifically the telnetd daemon. For over a decade (since 2015, to be precise), this code has harbored a logic bomb that turns the concept of "Input Validation" into a punchline. The daemon accepts environment variables from the client to set up the session. It takes these variables and hands them over to the system login binary. The problem? It hands them over a little too trustingly.

This is a "Deep Dive," so we aren't just going to say "patch it." We are going to dissect the corpse. We are going to look at how a single string manipulation error allows an unauthenticated attacker to become God (root) on a target system with a command that fits in a tweet.

To understand this exploit, you have to understand how telnet negotiates a session. Per RFC 1408 and RFC 1572, the Telnet protocol supports an option called NEW-ENVIRON. This allows the client and server to exchange environment variables. Ideally, this is used for harmless things like setting your TERM type (e.g., xterm-256color) or your DISPLAY.

However, it also allows sending the USER variable. The idea is convenience: if my local username is alice, the server can see that and pre-fill the login prompt with alice, waiting for my password. GNU telnetd takes this value and constructs a command line to invoke the /bin/login binary.

The root cause (CWE-88: Argument Injection) is that telnetd treats the USER variable as a raw string to be concatenated into a command template, rather than a distinct data payload. It assumes the user will send a name like bob or admin.

But what if the user sends -f root? The daemon constructs the command. It doesn't know that -f is a flag. It just knows it's a string. It passes this string to execve (or internal execution logic) which calls /bin/login. The login binary parses its arguments. It sees -f. On most Linux systems (util-linux, shadow), -f stands for force. It means: "The user is already authenticated, don't ask for a password."

It is the security equivalent of a bank vault guard asking, "Are you allowed to be here?" and you replying, "Yes, I'm the CEO," and the guard simply saying, "Okay, come on in," without checking an ID. The daemon inadvertently injected an argument that changed the behavior of the target program, rather than just supplying data to it.

Let's look at the source code. The vulnerability was introduced back in March 2015 and affects versions 1.9.3 through 2.7. The smoking gun is found in telnetd/telnetd.c and telnetd/utility.c.

In telnetd/telnetd.c, we define a template for how login should be invoked. Look closely at the format string:

/* Template command line for invoking login program.  */
char *login_invocation =
#else /* !SOLARIS */
  PATH_LOGIN " -p -h %h %?u{-f %u}{%U}"
#endif
  ;

This cryptic string %?u{-f %u}{%U} is a conditional. It roughly translates to:

• If authenticated via Kerberos (%u is set): Use -f <authenticated_user>.
• If NOT authenticated (%u is null): Use %U.

So, what is %U? Let's check telnetd/utility.c, where the expansion happens:

/* telnetd/utility.c */
char *
_var_short_name (struct line_expander *exp)
{
  // ... code ...
  switch (*exp->cp++)
    {
    // ...
    case 'U':
      /* CRITICAL FLAW: Reads env var directly without sanitization */
      return getenv ("USER") ? xstrdup (getenv ("USER")) : xstrdup ("3");
    // ...
    }
}

There is zero sanitation here. No check for hyphens. No check for special characters. It takes getenv("USER")—which is controlled by the attacker via the Telnet handshake—and returns it.

If the attacker sets USER to -f root, the expansion in login_invocation transforms:

PATH_LOGIN " -p -h %h %U"

Into:

/bin/login -p -h 192.168.1.50 -f root

The login binary receives -f as a distinct argument because of how the string is tokenized during the execution setup. The game is over before it began.

Exploiting this does not require heap feng shui, return-oriented programming, or complex shellcode. It requires a basic understanding of environment variables. The attack vector relies on the client (you) telling the server (the victim) what your username is.

Most telnet clients (like the standard Linux client) support the -l flag to set the user, or the -a flag to auto-login using current environment variables. However, to ensure the payload is passed exactly as we want, we can simply set the environment variable in our local shell before calling the client.

The Attack Chain

1. Attacker sets local USER environment variable to -f root.
2. Attacker connects to the victim telnetd server.
3. Telnet Protocol negotiates environment. Client sends USER value.
4. Victim Daemon constructs command: /bin/login ... -f root.
5. System Login sees -f root, assumes the daemon has already verified credentials.
6. Result: Instant root shell.

Proof of Concept

# The "Hacker" Command
USER='-f root' telnet -a target.example.com
 
# Or depending on your telnet client version:
telnet -l '-f root' target.example.com

Visually, the flow looks like this:

This works because telnetd runs as root (to bind port 23 and spawn processes), so when it executes login, it does so with the privileges required to tell login what to do. The -f flag is a feature designed for this exact scenario (trusting a daemon), but the vulnerability is that the user controls the trusted flag.

The CVSS score is 9.8/10 for a reason. This is the "Royal Flush" of vulnerabilities:

1. Remote: Exploitable over the network.
2. Unauthenticated: No valid credentials needed.
3. Low Complexity: A script kiddie can do it.
4. High Privileges: You get root, not www-data.

If you have a server exposing telnetd to the internet (according to Shodan, there are still hundreds of thousands), this is game over. An attacker can delete your filesystem, install ransomware, pivot to your internal network, or turn your server into a crypto-miner within seconds of connection.

While telnet is rare in modern enterprise Windows/Linux environments, it is surprisingly common in embedded devices, industrial control systems (ICS), and legacy mainframes. These devices often run GNU Inetutils or BusyBox (though BusyBox has different implementations, this specific bug is GNU-centric). The real danger here is the "forgotten" infrastructure—the dusty server in the closet that controls the HVAC system.

The remediation is straightforward, but it comes with a dose of reality.

Strategy 1: The "Nuke It" Approach (Recommended)

Stop using Telnet. Uninstall telnetd. Block port 23 at the firewall. Use SSH. There is almost zero justification for running an unencrypted, text-based remote shell service in 2026. If you must use it for legacy reasons, wrap it in a VPN and strictly limit access.

Strategy 2: Patching

If you are the maintainer of a legacy distro or an embedded system, you must apply the patches released by the GNU Inetutils team. The fix involves sanitizing the input before it is expanded.

The patch effectively checks the USER environment variable. If it detects that the value creates a new argument (starts with a hyphen or contains suspicious characters), it should reject it or sanitize it.

The Patch Logic (Simplified):

/* Pseudo-code of the fix */
val = getenv("USER");
if (val && *val == '-') {
    // Malicious user trying to inject flags
    val = "unknown"; 
}

Additionally, a robust fix involves changing how login is invoked. Instead of constructing a string, telnetd should use an argument array where the username is explicitly delimited, for example using -- to signify the end of options:

/bin/login -p -h host -- <username>

This ensures that even if <username> is -f root, login treats it as a username literal string starting with a dash, rather than a flag.