CVE-2013-4566 in Red Hat
Summary
by MITRE
mod_nss 1.0.8 and earlier, when NSSVerifyClient is set to none for the server/vhost context, does not enforce the NSSVerifyClient setting in the directory context, which allows remote attackers to bypass intended access restrictions.
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Analysis
by VulDB Data Team • 01/11/2022
The vulnerability described in CVE-2013-4566 affects the mod_nss module version 1.0.8 and earlier, which is a module for the Apache HTTP Server that provides SSL/TLS capabilities using the Network Security Services library. This issue stems from a configuration inheritance problem where the NSSVerifyClient directive fails to properly enforce security settings when transitioning from server/vhost context to directory context. The vulnerability specifically manifests when NSSVerifyClient is set to none in the server or virtual host configuration, creating a potential security gap that allows unauthorized access.
The technical flaw resides in the module's handling of directive inheritance and context evaluation. When NSSVerifyClient is configured to none at the server or virtual host level, the module should enforce this setting across all subdirectories unless explicitly overridden. However, the vulnerability demonstrates that the module fails to properly validate or enforce this setting when directory-level configurations are processed. This creates a scenario where attackers can potentially bypass intended access controls by manipulating directory-level configurations, effectively circumventing the security measures that should be in place.
The operational impact of this vulnerability is significant for organizations relying on mod_nss for SSL/TLS termination and access control. Remote attackers can exploit this weakness to gain unauthorized access to protected resources that should be restricted based on client certificate validation requirements. The vulnerability essentially allows attackers to bypass authentication mechanisms that depend on client certificate verification, potentially leading to unauthorized data access, privilege escalation, or further compromise of the affected system. This issue particularly affects web applications that require mutual TLS authentication or certificate-based access control.
This vulnerability aligns with CWE-284, which describes improper access control in software systems, and represents a clear case of privilege escalation through configuration mismanagement. The flaw also relates to ATT&CK technique T1566, which involves social engineering through phishing or other means, as attackers could exploit this configuration weakness to bypass authentication mechanisms. Organizations using mod_nss should implement immediate mitigations including updating to version 1.0.9 or later where this issue has been resolved, reviewing existing configurations to ensure proper enforcement of NSSVerifyClient directives, and implementing additional monitoring for unauthorized access attempts. The fix typically involves correcting the module's context handling logic to ensure that server-level settings are properly enforced even when directory-level configurations are present, thereby maintaining the intended security posture across all access control contexts.