CVE-1999-0990 in GDM
Summary
by MITRE
Error messages generated by gdm with the VerboseAuth setting allows an attacker to identify valid users on a system.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-0990 represents a significant authentication information disclosure flaw within the GNOME Display Manager gdm. This issue arises from the implementation of the VerboseAuth setting, which when enabled causes the system to provide detailed error messages during the authentication process. The flaw allows attackers to distinguish between valid and invalid user accounts through subtle differences in error responses, creating a reconnaissance opportunity that undermines the security of the authentication system. This type of vulnerability falls under the category of information disclosure, where system behavior reveals sensitive data about the underlying infrastructure.
The technical mechanism behind this vulnerability involves the gdm service's handling of authentication requests when the VerboseAuth option is activated. Instead of providing generic error messages for all authentication failures, the system generates specific responses that contain enough information to allow an attacker to determine whether a username exists in the system. This occurs because valid user accounts typically produce different error message patterns compared to invalid accounts, particularly when authentication fails due to incorrect passwords versus non-existent accounts. The difference in error message content provides attackers with a means to enumerate valid users through systematic testing, effectively creating a user enumeration attack vector.
The operational impact of this vulnerability extends beyond simple information disclosure, as it enables attackers to conduct targeted authentication attacks against valid accounts. Once an attacker has identified valid usernames through this method, they can focus subsequent password guessing or brute force attempts against those specific accounts, dramatically increasing the effectiveness of credential-based attacks. This vulnerability particularly affects systems where gdm is used as the primary display manager, which includes many linux desktop environments and server configurations that rely on GNOME-based authentication. The attack vector aligns with techniques described in the attack pattern taxonomy under the category of credential access and reconnaissance activities.
The security implications of this vulnerability are substantial as it violates fundamental security principles of least privilege and information hiding. The flaw essentially provides attackers with a reconnaissance tool that operates at the authentication layer, allowing them to bypass traditional network scanning techniques. According to CWE classification, this represents a CWE-209: Information Exposure Through an Error Message, where error handling mechanisms inadvertently reveal sensitive information. The vulnerability also maps to ATT&CK technique T1078.002: Valid Accounts, as it enables attackers to identify and target valid accounts for further exploitation. Organizations using gdm with verbose authentication enabled face increased risk of successful credential compromise attacks, particularly when combined with other reconnaissance techniques.
Mitigation strategies for this vulnerability involve disabling the VerboseAuth setting within the gdm configuration, which prevents the system from generating detailed error messages during authentication failures. System administrators should also implement account lockout mechanisms, enforce strong password policies, and consider implementing additional authentication layers such as two-factor authentication to reduce the impact of successful user enumeration. Network-level protections including rate limiting and intrusion detection systems can help detect and prevent automated user enumeration attempts. Regular security audits should verify that authentication systems are not providing unnecessary information in error messages, aligning with security best practices outlined in standards such as NIST SP 800-53 and ISO 27001. The vulnerability demonstrates the importance of proper error handling design in security-critical systems, where seemingly innocuous features can create significant attack surface opportunities.