CVE-2001-1476 in SSH
Summary
by MITRE
SSH before 2.0, with RC4 encryption and the "disallow NULL passwords" option enabled, makes it easier for remote attackers to guess portions of user passwords by replaying user sessions with certain modifications, which trigger different messages depending on whether the guess is correct or not.
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Analysis
by VulDB Data Team • 11/17/2024
This vulnerability exists in Secure Shell implementations prior to version 2.0 and specifically affects configurations using RC4 encryption with the "disallow NULL passwords" option enabled. The flaw stems from insufficient session replay protection mechanisms that allow attackers to exploit differences in system responses when attempting password guesses. The vulnerability operates through a timing and response analysis attack vector where the system provides distinguishable feedback based on password correctness, enabling password recovery through iterative guessing attempts.
The technical implementation flaw involves the protocol's handling of authentication responses during session replay scenarios. When an attacker replays a session with modified password attempts, the system generates different error messages or response patterns depending on whether the guessed portion matches the actual password. This differential response behavior creates a side-channel information leak that can be systematically exploited to reconstruct user passwords. The vulnerability is particularly dangerous because it leverages the RC4 encryption algorithm's weaknesses combined with the specific configuration option that should provide security but instead creates an attack surface.
The operational impact of this vulnerability is significant for systems utilizing older SSH implementations. Attackers can systematically guess password portions by observing response variations during replay attacks, potentially leading to complete password compromise. This vulnerability affects the fundamental authentication security model of SSH implementations and can result in unauthorized system access, privilege escalation, and data breaches. The attack requires minimal resources and can be automated, making it particularly dangerous for environments with weak password policies or high-value targets.
Mitigation strategies should focus on upgrading to SSH version 2.0 or later implementations that address the session replay and response consistency issues. Organizations must disable RC4 encryption where possible and ensure that authentication responses are consistent regardless of password correctness. The implementation should also include proper session management and response normalization to prevent side-channel information leakage. Security configurations should enforce strong password policies and implement account lockout mechanisms to reduce the effectiveness of iterative guessing attacks. This vulnerability aligns with CWE-200 (Information Exposure) and represents a classic example of how improper error handling can create security weaknesses that violate the principle of least privilege and secure by default configurations.
The attack pattern follows ATT&CK technique T1110.001 (Brute Force: Password Guessing) and demonstrates how protocol-level weaknesses can be exploited to bypass authentication mechanisms. Systems should implement proper rate limiting and account lockout procedures to prevent automated exploitation attempts. Regular security audits should verify that SSH implementations properly handle authentication responses and do not expose information that could aid in password recovery. The vulnerability highlights the importance of proper protocol design and the need for security considerations in all aspects of cryptographic implementation and authentication flow management.