Telnet Authentication Bypass Vulnerability Exposes Devices to Complete Takeover
A critical vulnerability in the Telnet protocol, tracked as CVE-2026-24061, has been discovered that allows attackers to bypass authentication and gain complete control over devices. This 11-year-old vulnerability affects many devices that are no longer supported and will not receive firmware updates, making them particularly vulnerable to exploitation. The vulnerability is especially concerning because it is trivial to exploit remotely, and many IoT and embedded devices continue to use Telnet for debugging purposes. As a result, devices with Telnet open are in immediate danger of being compromised. The vulnerability affects devices that use the GNU network utilities package, which is shipped with all Linux and other UNIX-based systems.
The vulnerability can be exploited by sending a carefully crafted USER environment value to the Telnet server, which then invokes the /usr/bin/login command with the value as the last parameter. This allows the attacker to bypass normal authentication processes and gain access to the device as the root user. The exploit can be achieved with a simple command: USER='-f root' telnet -a [host_ip]. This not only works against remote systems but can also serve as a privilege escalation exploit on the local machine if the Telnet service is running. The attack does not require any special privileges, as low-privileged users can typically initiate Telnet connections. The fact that Telnet is part of the GNU network utilities package, which is widely used, makes this vulnerability particularly widespread.
The implications of this vulnerability are significant, and organizations should take immediate action to mitigate the risk. The vulnerability has already been seen being exploited in the wild, with malicious traffic monitoring service GreyNoise reporting attempts to exploit the vulnerability. To mitigate the risk, users are advised to deploy the patch as soon as possible or update to a patched version offered by their distribution. As a temporary mitigation, users can disable the Telnet service entirely or filter access to it to only allow white-listed IP addresses. Organizations should also scan their networks and immediately isolate and firewall Telnet-enabled devices. The fact that this vulnerability can be exploited with such ease and has the potential to affect so many devices makes it a high-priority concern for cybersecurity professionals.
Attack Surface
IoT, Endpoint, Server OS
Tactics
Initial Access, Privilege Escalation, Lateral Movement
Techniques
- T1078 – Valid Accounts
- T1068 – Exploitation for Privilege Escalation
SuperPRO's Threat Countermeasures Procedures
- Disable the Telnet service entirely to prevent exploitation
- Filter access to the Telnet service to only allow white-listed IP addresses
- Deploy the patch as soon as possible or update to a patched version offered by the distribution
- Use a more secure protocol such as SSH instead of Telnet
- Scan networks and immediately isolate and firewall Telnet-enabled devices
- Monitor for suspicious Telnet activity and implement intrusion detection systems to detect potential exploitation attempts
- Implement a vulnerability management program to identify and remediate vulnerabilities in a timely manner