CVE-1999-0740 in Linux
Summary
by MITRE
Remote attackers can cause a denial of service on Linux in.telnetd telnet daemon through a malformed TERM environmental variable.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability identified as CVE-1999-0740 represents a critical denial of service flaw within the in.telnetd telnet daemon implementation on linux systems. This issue specifically targets the handling of terminal environment variables during remote telnet sessions, creating a scenario where malicious actors can disrupt system availability without requiring authentication or elevated privileges. The vulnerability exists in the daemon's processing of the TERM environmental variable, which is used to specify the terminal type for remote sessions. When a malformed TERM variable is transmitted during the telnet connection establishment process, the daemon fails to properly validate or sanitize this input, leading to system instability and potential service disruption.
The technical root cause of this vulnerability stems from inadequate input validation within the telnet daemon's environment variable processing logic. According to CWE classification, this represents a weakness in input validation and improper error handling within network services. The flaw occurs during the initial session setup phase when the telnet daemon receives and processes environmental variables from remote clients. The daemon's failure to properly handle malformed terminal type specifications creates a condition where buffer overflows or memory corruption can occur, ultimately leading to the daemon's termination or system resource exhaustion. This vulnerability operates at the application layer and specifically targets the telnet protocol implementation, making it particularly dangerous given the widespread deployment of telnet services in network infrastructure.
The operational impact of CVE-1999-0740 extends beyond simple service disruption to potentially compromise network availability and system integrity. Attackers can exploit this vulnerability to repeatedly cause denial of service conditions, effectively preventing legitimate users from establishing telnet sessions to affected systems. The vulnerability demonstrates characteristics aligned with ATT&CK technique T1499.004 for network denial of service, as it specifically targets network services to prevent legitimate access. In enterprise environments where telnet services remain operational, this vulnerability could be leveraged to disrupt critical administrative access points, potentially forcing organizations to rely on alternative access methods or temporarily disable services. The remote nature of the exploit means that attackers can target vulnerable systems from anywhere on the network without requiring physical access or prior authentication credentials.
Mitigation strategies for this vulnerability should focus on immediate patching and configuration hardening measures. System administrators should prioritize applying security updates that address the input validation flaws in the telnet daemon implementation, with particular attention to the handling of environmental variables. The most effective approach involves disabling or removing the telnet service entirely, as modern security practices strongly recommend against using telnet for remote administration due to its plaintext transmission of credentials and inherent security weaknesses. Organizations should implement network segmentation to limit exposure of telnet services and consider deploying SSH as a secure alternative for remote administration tasks. Additionally, monitoring systems should be configured to detect unusual patterns of telnet connection attempts that might indicate exploitation attempts, while network access control lists can be used to restrict access to telnet services to trusted networks only. The vulnerability also underscores the importance of input validation and proper error handling in network services, as recommended by security frameworks such as the OWASP Top Ten and NIST cybersecurity guidelines.