CVE-2011-4966 in FreeRADIUS
Summary
by MITRE
modules/rlm_unix/rlm_unix.c in FreeRADIUS before 2.2.0, when unix mode is enabled for user authentication, does not properly check the password expiration in /etc/shadow, which allows remote authenticated users to authenticate using an expired password.
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Analysis
by VulDB Data Team • 12/30/2021
The vulnerability identified as CVE-2011-4966 resides within the FreeRADIUS authentication system, specifically in the modules/rlm_unix/rlm_unix.c component. This issue affects versions prior to 2.2.0 and manifests when the unix authentication mode is enabled for user authentication processes. The core problem involves a critical failure in the password expiration validation mechanism that operates within the /etc/shadow file, which serves as the primary repository for user password information including expiration dates and other security attributes. The flaw represents a significant oversight in the authentication flow where the system fails to properly verify that user credentials remain valid according to the established password policies.
The technical implementation of this vulnerability stems from insufficient validation logic within the rlm_unix module that interfaces with the Unix authentication subsystem. When FreeRADIUS operates in unix mode, it typically consults the /etc/shadow file to determine user account status including password expiration dates. However, the vulnerable code path fails to perform adequate checks against the password expiration fields within this file, specifically the expiration date field that indicates when a password should no longer be accepted. This oversight means that even when a user's password has officially expired according to system policies, the authentication process continues to accept the credentials, effectively bypassing the intended security controls.
From an operational perspective, this vulnerability creates a serious security risk for organizations relying on FreeRADIUS for network access control. Remote authenticated users who possess valid but expired credentials can continue to authenticate successfully, undermining the fundamental security principle that expired passwords should not be accepted for access. This situation effectively provides a backdoor for continued access even after password expiration policies have been enforced, potentially allowing malicious actors who have obtained valid credentials to maintain unauthorized access beyond their intended time period. The impact extends to compliance requirements where password expiration policies are mandated for security governance and audit purposes.
The vulnerability aligns with CWE-284, which addresses improper access control, and represents a specific instance where access control mechanisms fail to properly validate account status before granting authentication. From an ATT&CK framework perspective, this weakness maps to privilege escalation and credential access tactics, as it allows attackers to maintain access using expired credentials that should have been invalidated. Organizations implementing FreeRADIUS with unix authentication mode face significant risk of unauthorized access persistence, particularly in environments where strict password policies are enforced. The vulnerability demonstrates a critical failure in the authentication validation pipeline where the system should have rejected expired credentials but instead permitted continued access, creating a security gap that could be exploited for extended unauthorized access periods.
Mitigation strategies for CVE-2011-4966 require immediate implementation of the FreeRADIUS 2.2.0 release or subsequent versions that contain the corrected validation logic. System administrators should also conduct comprehensive audits of their authentication policies and ensure that all FreeRADIUS installations are updated to versions that properly validate password expiration status. Additionally, organizations should implement monitoring solutions to detect unusual authentication patterns that might indicate exploitation of this vulnerability, particularly around the timing of expired credential usage. The fix addresses the root cause by implementing proper validation of the password expiration fields within /etc/shadow, ensuring that authentication requests are rejected when user accounts have expired according to established security policies.