CVE-2021-3547 in Core Library
Summary
by MITRE • 07/12/2021
OpenVPN 3 Core Library version 3.6 and 3.6.1 allows a man-in-the-middle attacker to bypass the certificate authentication by issuing an unrelated server certificate using the same hostname found in the verify-x509-name option in a client configuration.
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Analysis
by VulDB Data Team • 07/15/2021
The vulnerability described in CVE-2021-3547 represents a critical flaw in the OpenVPN 3 Core Library version 3.6 and 3.6.1 that fundamentally undermines the certificate-based authentication mechanism designed to secure virtual private network connections. This weakness enables a man-in-the-middle attacker to exploit the certificate verification process by presenting an unrelated server certificate that happens to contain the same hostname as specified in the verify-x509-name option within client configurations. The attack exploits a fundamental mismatch between the hostname validation logic and the certificate verification procedures, creating a scenario where authentication can be bypassed without proper cryptographic validation.
The technical flaw stems from the improper handling of certificate verification when multiple certificates with identical hostnames are present in the certificate chain or when the verify-x509-name option fails to enforce strict certificate validation. This vulnerability falls under the category of weak certificate validation and authentication bypass, specifically aligning with CWE-295 which addresses improper certificate validation and CWE-310 which covers cryptographic issues related to authentication. The flaw operates at the core authentication layer of the OpenVPN implementation, where the system should be enforcing strict certificate chain validation but instead allows for certificate substitution based on hostname matching alone.
The operational impact of this vulnerability is severe as it completely undermines the security assurances provided by certificate-based authentication in OpenVPN deployments. Any organization relying on OpenVPN 3 Core Library for secure remote access is at risk of having their communications intercepted and potentially manipulated by attackers who can present a different certificate with the same hostname. This vulnerability affects not only the confidentiality of data transmitted through the VPN but also compromises the integrity of the connection, as attackers can establish fraudulent connections that appear legitimate to clients. The attack can be executed without requiring advanced technical skills or specific access to the network, making it particularly dangerous in environments where OpenVPN is widely deployed for remote worker access or secure communications.
Organizations affected by this vulnerability should immediately update to patched versions of OpenVPN 3 Core Library, specifically version 3.6.2 or later, which addresses the certificate validation flaw. Additional mitigations include implementing strict certificate pinning mechanisms, deploying additional network monitoring to detect unusual certificate exchanges, and reviewing existing client configurations to ensure the verify-x509-name option is properly configured with additional validation criteria beyond simple hostname matching. From an ATT&CK perspective, this vulnerability maps to techniques involving credential access through authentication bypass and privilege escalation, as the attacker can effectively impersonate legitimate VPN servers and gain unauthorized access to protected network resources. Network administrators should also consider implementing certificate transparency monitoring and regular security audits of their OpenVPN deployments to detect and remediate similar vulnerabilities before they can be exploited in real-world scenarios.