CVE-2026-24061 in Inetutils
Summary
by MITRE • 01/21/2026
telnetd in GNU Inetutils through 2.7 allows remote authentication bypass via a "-f root" value for the USER environment variable.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 05/01/2026
The vulnerability identified as CVE-2026-24061 affects telnetd implementations within GNU Inetutils versions 2.7 and earlier, presenting a critical authentication bypass flaw that could allow remote attackers to gain unauthorized system access. This vulnerability specifically exploits the handling of the USER environment variable, where an attacker can supply a malicious value of "-f root" to circumvent the normal authentication process and establish a telnet session with root privileges. The flaw resides in the telnet daemon's improper validation of user input, particularly when processing environment variables during the authentication handshake process.
The technical implementation of this vulnerability stems from inadequate input sanitization and validation within the telnetd service. When a client connects to the telnet daemon, the service attempts to authenticate the user by processing various environment variables including USER. The vulnerability occurs because the implementation fails to properly validate or sanitize the USER variable value, allowing crafted input to be interpreted in a way that bypasses authentication mechanisms. This type of flaw falls under CWE-20, which addresses improper input validation, and specifically relates to CWE-502, dealing with deserialization of untrusted data, though in this case it manifests through environment variable handling rather than data parsing. The vulnerability represents a classic case of insecure parameter handling where environment variables are directly used in system calls without proper sanitization.
The operational impact of this vulnerability is severe as it allows remote attackers to execute commands with root privileges on affected systems. An attacker who can establish a telnet connection to a vulnerable system can immediately bypass authentication and gain full administrative access without requiring valid credentials. This creates a significant risk for systems running GNU Inetutils telnetd services, particularly in environments where telnet is still enabled and accessible over the network. The vulnerability is particularly dangerous because it does not require any special privileges or local access to exploit, making it a serious concern for network security. The threat model aligns with ATT&CK technique T1075, which covers "Pass the Hash" and similar credential bypass techniques, though this vulnerability operates at a different layer by directly manipulating the authentication flow through environment variable manipulation.
The exploitation of this vulnerability demonstrates a fundamental flaw in how the telnet daemon processes user authentication parameters, where environment variables are treated as trusted input without proper validation. This represents a critical security oversight in the software design, as the daemon should never trust user-supplied environment variables in authentication contexts. The vulnerability is particularly concerning in enterprise environments where legacy telnet services may still be operational, as it provides an immediate path to privilege escalation for attackers who can reach the telnet service over the network. Organizations should consider this vulnerability in their risk assessment frameworks, particularly when evaluating the security posture of systems that maintain legacy network services. The flaw underscores the importance of proper input validation and the principle of least privilege in service implementations, as well as the necessity of regularly updating and reviewing network services for security vulnerabilities. Remediation efforts should include immediate patching of affected GNU Inetutils versions, disabling telnet services where possible, and implementing network segmentation to limit access to these vulnerable services.