Description

A critical security vulnerability has been disclosed in the GNU InetUtils telnet daemon (telnetd) that allows remote attackers to gain unauthorized root access to affected systems. Tracked as CVE-2026-24061, the flaw impacts telnetd versions ranging from 1.9.3 to 2.7 and has remained undetected for nearly 11 years. Successful exploitation enables an attacker to bypass the normal authentication process entirely and obtain a privileged shell over the network. Due to its severity and ease of exploitation, the vulnerability has been assigned a CVSS score of 9.8 (Critical), making it a significant risk for systems still running telnet services. The root cause of the issue lies in improper input handling of environment variables by telnetd. Specifically, the daemon incorrectly processes the USER environment variable supplied by a remote client and passes it directly to the system login binary, which executes with root privileges. By crafting a malicious value (such as a login flag that suppresses authentication), an attacker can manipulate the login process to automatically authenticate as the root user without providing valid credentials. This flaw stems from a lack of argument sanitization and unsafe reliance on legacy login behavior. To mitigate this risk, organizations are strongly advised to disable telnetd entirely wherever possible, as Telnet is an outdated and insecure protocol. Systems that require telnetd must be immediately patched using updated packages provided by their Linux distribution. Network access to Telnet should be strictly limited, and administrators should transition to secure alternatives like SSH. Continuous monitoring, system audits, and removal of legacy services are recommended to reduce exposure to similar vulnerabilities.