CVE-1999-0073 in IRIX
Summary
by MITRE
Telnet allows a remote client to specify environment variables including LD_LIBRARY_PATH, allowing an attacker to bypass the normal system libraries and gain root access.
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Analysis
by VulDB Data Team • 02/24/2026
The vulnerability described in CVE-1999-0073 represents a critical security flaw in the Telnet protocol implementation that enables remote code execution through environment variable manipulation. This issue specifically affects the telnet daemon's handling of client-specified environment variables during the connection establishment process. When a remote client connects to a telnet server, the server accepts and processes environment variables sent by the client without proper validation or sanitization, creating a significant attack vector for privilege escalation.
The technical exploitation of this vulnerability relies on the ability of an attacker to inject malicious values into the LD_LIBRARY_PATH environment variable, which is a critical system variable that determines the order in which shared libraries are loaded by the dynamic linker. When the telnet daemon processes this variable, it allows the attacker to manipulate the library loading sequence, potentially causing the system to load malicious versions of system libraries from attacker-controlled directories. This manipulation can lead to code execution with elevated privileges, as the attacker's malicious libraries can be loaded in place of legitimate system libraries during critical operations.
This vulnerability directly maps to CWE-250, which describes the improper handling of environment variables, and represents a classic example of privilege escalation through environment variable manipulation. The operational impact of this vulnerability is severe as it allows attackers to bypass normal system security controls and gain root access to affected systems. The vulnerability affects systems running telnet servers that do not properly validate environment variables received from remote clients, making it particularly dangerous in networked environments where telnet services are exposed to untrusted networks. The attack requires minimal privileges to initiate and can be automated, making it a preferred method for attackers seeking to establish persistent access to target systems.
The exploitation of this vulnerability demonstrates the broader security implications of trusting remote input without proper validation, particularly in service-oriented applications that handle user-provided data. Organizations should implement immediate mitigations including disabling telnet services in favor of secure alternatives such as SSH, implementing proper environment variable validation on telnet servers, and ensuring that system libraries are protected against unauthorized modification. The vulnerability also highlights the importance of following security best practices outlined in the MITRE ATT&CK framework, specifically in the privilege escalation and defense evasion techniques categories. System administrators should conduct comprehensive security audits to identify all systems running telnet services and ensure proper patching or service removal to prevent exploitation. Additionally, network segmentation and access control measures should be implemented to limit exposure of telnet services to trusted networks only, reducing the attack surface for potential exploitation of this and similar vulnerabilities.