CVE-2011-0438 in nss-pam-ldapd
Summary
by MITRE
nslcd/pam.c in the nss-pam-ldapd 0.8.0 PAM module returns a success code when a user is not found in LDAP, which allows remote attackers to bypass authentication.
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Analysis
by VulDB Data Team • 02/09/2019
The vulnerability identified as CVE-2011-0438 resides within the nss-pam-ldapd 0.8.0 PAM module, specifically in the nslcd/pam.c component. This flaw represents a critical authentication bypass vulnerability that fundamentally undermines the security posture of systems relying on LDAP-based user authentication. The issue manifests when the PAM module encounters a user lookup operation that fails to find a corresponding user account within the LDAP directory structure, yet still returns a success code instead of an appropriate failure indicator.
This technical flaw operates at the intersection of identity management and authentication controls, creating a scenario where unauthorized access becomes possible through systematic user enumeration techniques. The vulnerability stems from improper error handling within the PAM module's user validation logic, where the module fails to distinguish between a successful user lookup and a failed lookup when the user simply does not exist in the directory service. This misclassification allows attackers to exploit the system by attempting authentication with various user accounts and observing the inconsistent responses.
From an operational impact perspective, this vulnerability enables remote attackers to perform user enumeration attacks against systems using LDAP authentication, potentially leading to account compromise and unauthorized system access. The flaw directly violates fundamental security principles by providing attackers with information that should remain hidden, specifically revealing which user accounts exist within the LDAP directory. This information can be leveraged to construct targeted attacks, identify valid accounts for brute force attempts, or facilitate further reconnaissance activities within the network environment.
The vulnerability aligns with CWE-287, which addresses improper authentication issues, and demonstrates characteristics consistent with ATT&CK technique T1110.003 for credential access through brute force methods. Attackers can systematically test user accounts and exploit the inconsistent return codes to identify valid accounts, effectively bypassing the intended authentication controls. This creates a significant risk for organizations relying on LDAP-based authentication systems, particularly in environments where proper access controls and monitoring are not adequately implemented.
Mitigation strategies for this vulnerability include immediate patching of the nss-pam-ldapd module to version 0.8.1 or later, which contains the necessary fixes for proper error handling. Organizations should also implement additional monitoring and logging mechanisms to detect anomalous authentication patterns and user enumeration attempts. Network segmentation and access control measures should be strengthened to limit the attack surface, while regular security assessments should verify that authentication modules are properly configured and that appropriate error handling is in place to prevent information leakage through authentication responses.