CVE-2001-1459 in OpenSSH
Summary
by MITRE
OpenSSH 2.9 and earlier does not initiate a Pluggable Authentication Module (PAM) session if commands are executed with no pty, which allows local users to bypass resource limits (rlimits) set in pam.d.
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Analysis
by VulDB Data Team • 09/25/2025
The vulnerability identified as CVE-2001-1459 represents a critical security flaw in OpenSSH versions 2.9 and earlier that fundamentally undermines the integrity of access control mechanisms through improper handling of Pluggable Authentication Module sessions. This weakness occurs specifically when authentication occurs without a pseudo-terminal allocation, creating an exploitable gap in the security model that was prevalent during the early 2000s era of Unix-like system administration. The flaw stems from the incorrect assumption that certain security policies should only apply when interactive sessions are established through a terminal interface, thereby leaving automated or non-interactive command execution vulnerable to privilege escalation attacks.
The technical implementation of this vulnerability resides in the authentication flow of OpenSSH where the system fails to properly initialize PAM sessions for non-pty command execution contexts. When users execute commands without a pseudo-terminal, the system bypasses the normal PAM session establishment process that would typically enforce resource limits defined in pam.d configuration files. This misconfiguration allows local users to circumvent the rlimits imposed by PAM, effectively removing the boundaries that would normally constrain resource consumption during authentication and session management. The underlying mechanism operates through the failure to invoke the pam_start function with appropriate session management parameters when no terminal is present, leaving the authentication framework in an incomplete state.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass broader system stability and resource management concerns. Local attackers can exploit this flaw to bypass resource constraints that were specifically designed to prevent denial-of-service attacks, allowing them to consume excessive system resources without restriction. This creates potential for system instability and can be leveraged as part of a broader attack strategy to exhaust system resources, making it particularly dangerous in multi-user environments where resource contention could affect overall system performance. The vulnerability particularly affects systems where administrators rely on PAM-based resource limits to maintain system integrity and prevent abuse of authentication mechanisms.
The security implications of this vulnerability align with CWE-284, which addresses improper access control in authentication mechanisms, and can be categorized under ATT&CK technique T1078 for valid accounts and T1496 for resource hijacking. Organizations implementing OpenSSH services in environments where local users might have elevated privileges face significant risk from this vulnerability, as it effectively nullifies the security controls that PAM is designed to enforce. The exploitation requires local access and knowledge of the specific OpenSSH version, but the impact is substantial enough to warrant immediate attention from system administrators and security teams responsible for maintaining secure authentication infrastructure.
Mitigation strategies for this vulnerability require immediate patching of OpenSSH installations to versions that properly implement PAM session management regardless of terminal allocation status. System administrators should also review and audit existing PAM configurations to ensure that resource limits are properly enforced across all authentication contexts, not just interactive sessions. Additional defensive measures include implementing monitoring for unusual resource consumption patterns and ensuring that access controls are consistently applied to both interactive and non-interactive command execution scenarios. The fix implemented in later OpenSSH versions addresses the root cause by ensuring that PAM session initialization occurs consistently regardless of whether a pseudo-terminal is present during authentication, thereby maintaining the integrity of resource management policies established through pam.d configuration files.