CVE-2011-2701 in FreeRADIUS
Summary
by MITRE
The ocsp_check function in rlm_eap_tls.c in FreeRADIUS 2.1.11, when OCSP is enabled, does not properly parse replies from OCSP responders, which allows remote attackers to bypass authentication by using the EAP-TLS protocol with a revoked X.509 client certificate.
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Analysis
by VulDB Data Team • 01/12/2025
The vulnerability described in CVE-2011-2701 represents a critical authentication bypass flaw in FreeRADIUS version 2.1.11 that specifically affects the EAP-TLS protocol implementation. This issue resides within the ocsp_check function located in the rlm_eap_tls.c module, where the system fails to properly validate OCSP (Online Certificate Status Protocol) responses from certificate authorities. The flaw enables malicious actors to exploit the certificate validation process by presenting revoked X.509 client certificates that should normally be rejected, thereby allowing unauthorized access to protected network resources through the EAP-TLS authentication mechanism.
The technical root cause of this vulnerability stems from inadequate input validation and parsing of OCSP responses within the FreeRADIUS server implementation. When OCSP is enabled for certificate validation, the system should rigorously verify the status of client certificates against the responder's response to ensure they have not been revoked. However, the ocsp_check function contains a parsing flaw that permits malformed or improperly structured OCSP responses to be accepted as valid, effectively bypassing the certificate revocation check. This parsing weakness creates a condition where even if a certificate has been revoked by the issuing authority, the system may incorrectly accept it as valid due to the improper handling of the OCSP response data structure.
The operational impact of this vulnerability is severe and far-reaching within network security environments that rely on FreeRADIUS for EAP-TLS authentication. Attackers can exploit this flaw to gain unauthorized access to network resources that should only be accessible to legitimate users with valid certificates, potentially compromising entire network segments. The vulnerability specifically targets environments using EAP-TLS protocol implementations where certificate-based authentication is critical for security, such as wireless networks, VPN access, and enterprise network infrastructure. This allows attackers to establish connections using compromised or revoked certificates, undermining the fundamental security assumptions of certificate-based authentication systems and potentially enabling further lateral movement within the network.
Organizations utilizing FreeRADIUS 2.1.11 with OCSP enabled are particularly vulnerable to this attack vector, as the flaw affects the core certificate validation process that protects network access. The vulnerability aligns with CWE-20, which describes improper input validation, and represents a specific instance of how flawed parsing of security-critical responses can lead to authentication bypasses. From an ATT&CK framework perspective, this vulnerability maps to technique T1566.002 (Phishing via Service) and T1078.002 (Valid Accounts: Domain Accounts) as attackers can leverage this flaw to establish unauthorized access using compromised certificates. The security implications extend beyond simple access control, as this vulnerability could enable more sophisticated attacks including man-in-the-middle scenarios where attackers can impersonate legitimate users and potentially intercept or manipulate network traffic.
The recommended mitigation strategies include immediate upgrade to FreeRADIUS versions that contain patched implementations of the ocsp_check function, ensuring proper OCSP response validation, and implementing additional monitoring for anomalous authentication patterns. Organizations should also consider disabling OCSP validation temporarily while applying patches, implementing certificate lifecycle management processes to ensure timely certificate revocation, and deploying network monitoring solutions to detect potential exploitation attempts. Additionally, implementing certificate pinning mechanisms and maintaining detailed audit logs of authentication events can help detect when this vulnerability has been successfully exploited. The fix typically involves correcting the parsing logic to properly validate OCSP response structures and ensure that revoked certificate status information is correctly interpreted and enforced during the authentication process, thereby restoring the intended security controls of the EAP-TLS protocol implementation.