CVE-2002-1646 in SSH
Summary
by MITRE
SSH Secure Shell for Servers 3.0.0 to 3.1.1 allows remote attackers to override the AllowedAuthentications configuration and use less secure authentication schemes (e.g. password) than configured for the server.
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Analysis
by VulDB Data Team • 09/03/2025
The vulnerability described in CVE-2002-1646 represents a critical configuration flaw in SSH Secure Shell for Servers versions 3.0.0 through 3.1.1 that fundamentally undermines the security posture of affected systems. This vulnerability operates at the authentication layer of the SSH protocol, where it permits remote attackers to bypass the intended security controls that administrators have implemented to restrict authentication methods. The flaw specifically targets the AllowedAuthentications configuration parameter, which is designed to enforce strict authentication policies by limiting clients to only use approved authentication schemes such as public key authentication, which provides significantly stronger security than password-based methods.
The technical execution of this vulnerability exploits a weakness in the server's authentication validation process, allowing malicious actors to manipulate the authentication flow and force the system to accept less secure authentication mechanisms regardless of the administrator's explicit configuration. This occurs because the server fails to properly validate or enforce the authentication restrictions that have been configured, creating a pathway for attackers to downgrade the security level of the authentication process. The vulnerability essentially creates a backdoor that allows unauthorized users to bypass the intended security controls and potentially gain access using password-based authentication methods that are typically disabled for security reasons.
From an operational impact perspective, this vulnerability poses severe risks to organizations that rely on SSH for secure remote access and system administration. The ability to override authentication restrictions means that attackers can potentially exploit systems even when strong authentication methods like public key authentication have been configured and enforced. This creates a scenario where the security controls that administrators have put in place to protect their systems become ineffective, leading to potential unauthorized access, privilege escalation, and data compromise. The vulnerability directly violates the principle of least privilege and can enable attackers to move laterally within networks where SSH is used for administrative access.
The implications of this vulnerability extend beyond simple authentication bypass to encompass broader security compliance issues and risk management concerns. Organizations implementing SSH security policies based on the assumption that AllowedAuthentications would properly restrict authentication methods face significant exposure when this vulnerability is exploited. The flaw aligns with CWE-284, which addresses improper access control, and represents a clear violation of security configuration management principles. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and initial access through remote services, making it particularly dangerous for attackers seeking to establish persistent access to target systems.
Mitigation strategies for this vulnerability require immediate patching of affected SSH Secure Shell for Servers installations to version 3.2.0 or later, which contains the necessary fixes to properly enforce authentication restrictions. Organizations should also implement network segmentation and access controls to limit exposure of affected systems, while conducting thorough security audits to identify any potential exploitation attempts. Additional defensive measures include monitoring for unusual authentication patterns and ensuring that all SSH configurations are properly validated and tested before deployment. System administrators should also review and strengthen their overall SSH security posture, including implementing mandatory use of public key authentication and disabling password authentication entirely on systems where it is not absolutely required. The vulnerability demonstrates the critical importance of maintaining up-to-date security software and the potential consequences of configuration management failures in network security infrastructure.