CVE-2015-2091 in Mod-gnutls
Summary
by MITRE
The authentication hook (mgs_hook_authz) in mod-gnutls 0.5.10 and earlier does not validate client certificates when "GnuTLSClientVerify require" is set, which allows remote attackers to spoof clients via a crafted certificate.
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Analysis
by VulDB Data Team • 11/29/2024
The vulnerability identified as CVE-2015-2091 affects the mod-gnutls module version 0.5.10 and earlier, specifically within the authentication hook mechanism known as mgs_hook_authz. This flaw represents a critical security oversight in the handling of client certificate validation within the Apache HTTP Server environment. The vulnerability occurs when the GnuTLSClientVerify directive is set to "require" mode, which should theoretically mandate certificate validation but fails to properly enforce this requirement. The issue stems from insufficient validation logic in the authentication hook implementation that allows malicious actors to bypass the intended certificate verification process. This creates a scenario where unauthorized parties can present forged certificates and still gain authenticated access to systems that rely on this module for secure communication. The vulnerability directly impacts the fundamental security model of certificate-based authentication, undermining the trust assumptions that secure web services depend upon for access control and identity verification. From a cybersecurity perspective, this represents a failure in the principle of least privilege and proper authentication enforcement.
The technical root cause of CVE-2015-2091 resides in the improper implementation of the mgs_hook_authz function within the mod-gnutls module, which is designed to handle authentication callbacks for GnuTLS-enabled Apache servers. When the GnuTLSClientVerify directive is configured to require certificate validation, the module should enforce strict certificate verification including proper certificate chain validation, signature verification, and trust chain establishment. However, the vulnerability exists because the authentication hook fails to properly validate client certificates even when the configuration explicitly demands verification. This creates a condition where the system accepts client certificates without performing essential cryptographic checks that should occur during the authentication process. The flaw essentially creates a bypass mechanism that allows attackers to submit any certificate, including self-signed or maliciously crafted certificates, and still be authenticated as valid clients. The vulnerability can be categorized under CWE-295 as "Improper Certificate Validation" and aligns with ATT&CK technique T1552.001 for "Credentials from Password Stores" and T1078.002 for "Valid Accounts: Domain Accounts" when exploited in authentication contexts.
The operational impact of CVE-2015-2091 extends beyond simple authentication bypass, potentially enabling sophisticated attacks such as man-in-the-middle scenarios, unauthorized access to protected resources, and privilege escalation within systems that depend on certificate-based authentication. Attackers can exploit this vulnerability to impersonate legitimate clients, access restricted content, or gain unauthorized privileges within applications that rely on mod-gnutls for secure communication. The vulnerability is particularly concerning in environments where client certificates are used for mutual authentication, such as enterprise applications, secure web services, or API gateways that require client certificate validation. Organizations using affected versions of mod-gnutls may experience unauthorized access to sensitive data, service disruption, and potential data breaches. The vulnerability's exploitation does not require special privileges or complex attack vectors, making it particularly dangerous as it can be leveraged by relatively unsophisticated attackers. The impact is amplified in scenarios where certificate-based authentication is used as part of multi-factor authentication systems or where client certificates are used for access control to critical infrastructure components.
Mitigation strategies for CVE-2015-2091 primarily involve immediate upgrade to mod-gnutls version 0.5.11 or later, which contains the necessary patches to properly enforce client certificate validation. Organizations should also implement comprehensive monitoring to detect unauthorized certificate usage patterns and establish proper certificate lifecycle management processes. Security administrators should review all Apache configurations that utilize GnuTLSClientVerify directives and ensure proper certificate validation is enforced through alternative mechanisms while the upgrade is pending. Additionally, implementing network segmentation, access controls, and monitoring solutions can help detect and prevent exploitation attempts. The vulnerability highlights the importance of proper input validation and authentication enforcement in security-critical modules, emphasizing the need for regular security updates and vulnerability assessments. Organizations should also consider implementing certificate pinning mechanisms and additional authentication layers to provide defense-in-depth against similar vulnerabilities. The incident underscores the critical importance of maintaining up-to-date security software and proper security testing procedures to prevent such authentication bypass vulnerabilities from being exploited in production environments.