CVE-2007-2243 in OpenSSH
Summary
by MITRE
OpenSSH 4.6 and earlier, when ChallengeResponseAuthentication is enabled, allows remote attackers to determine the existence of user accounts by attempting to authenticate via S/KEY, which displays a different response if the user account exists, a similar issue to CVE-2001-1483.
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Analysis
by VulDB Data Team • 07/14/2021
The vulnerability described in CVE-2007-2243 represents a significant security flaw in OpenSSH versions 4.6 and earlier that affects the authentication process when ChallengeResponseAuthentication is enabled. This issue stems from the improper handling of authentication responses that reveal information about user account existence, creating a potential avenue for account enumeration attacks. The flaw specifically manifests when users attempt to authenticate using S/KEY challenge-response authentication methods, which should normally provide a consistent authentication interface regardless of whether the target account exists.
The technical implementation of this vulnerability lies in the differential response behavior of the SSH server when processing S/KEY authentication attempts. When ChallengeResponseAuthentication is enabled, the server provides distinct error messages or response patterns depending on whether the target user account exists within the system's user database. This information disclosure occurs because the authentication system does not normalize its response behavior across different account states, allowing attackers to distinguish between valid and invalid account names through careful analysis of the server's reaction time or error messages. The vulnerability essentially creates a side-channel attack vector where account existence becomes determinable through authentication attempts.
The operational impact of this vulnerability extends beyond simple account enumeration, as it provides attackers with critical information that can facilitate subsequent attacks. An attacker who can determine which user accounts exist on a system can then focus their efforts on those specific accounts, potentially leading to credential brute-forcing, password spraying attacks, or other targeted authentication attempts. This vulnerability particularly affects systems that rely on ChallengeResponseAuthentication for additional security layers, as it undermines the fundamental security principle that authentication systems should not leak information about their internal state through response variations. The issue is classified under CWE-200, which addresses Information Exposure, and aligns with ATT&CK technique T1078.004 for Valid Accounts, as it enables adversaries to identify legitimate user accounts for later exploitation.
Mitigation strategies for CVE-2007-2243 should focus on disabling ChallengeResponseAuthentication when it is not strictly required for the system's security posture. System administrators should implement the latest OpenSSH versions that address this vulnerability, as version 4.7 and later contain fixes that normalize authentication responses regardless of account existence. Additionally, organizations should consider implementing rate limiting mechanisms to prevent rapid enumeration attempts, deploy intrusion detection systems to monitor for suspicious authentication patterns, and ensure that authentication logs are properly monitored for unusual account access attempts. The vulnerability also underscores the importance of following security best practices such as implementing strong account lockout policies and using multi-factor authentication to reduce the risk of successful exploitation. Organizations should conduct regular security assessments to identify and remediate similar information disclosure vulnerabilities in their authentication infrastructure, as this type of flaw can significantly weaken overall security posture.