CVE-2005-2531 in OpenVPN
Summary
by MITRE
OpenVPN before 2.0.1, when running with "verb 0" and without TLS authentication, does not properly flush the OpenSSL error queue when a client fails certificate authentication to the server and causes the error to be processed by the wrong client, which allows remote attackers to cause a denial of service (client disconnection) via a large number of failed authentication attempts.
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Analysis
by VulDB Data Team • 06/11/2019
The vulnerability described in CVE-2005-2531 represents a critical flaw in OpenVPN versions prior to 2.0.1 that manifests when the server operates with verb 0 logging level and lacks TLS authentication mechanisms. This issue stems from improper error handling within the OpenSSL library integration, specifically concerning how error queue states are managed during certificate authentication failures. The flaw occurs when multiple client authentication attempts fail sequentially, creating a cascading effect where error states from one client inadvertently influence subsequent client connections. The root cause lies in the failure to properly clear the OpenSSL error queue after each authentication attempt, leading to erroneous error propagation between different client sessions. This mismanagement of error states creates a condition where legitimate authentication failures can be incorrectly attributed to other clients, causing unexpected disconnections and disrupting normal service operations.
The operational impact of this vulnerability extends beyond simple denial of service, as it enables attackers to exploit the error queue flushing mechanism through repeated failed authentication attempts. When an attacker systematically submits invalid certificate credentials, the server's improper error handling causes each failed attempt to pollute the shared OpenSSL error state, eventually leading to legitimate clients being disconnected due to incorrectly processed error conditions. This vulnerability operates at the intersection of cryptographic protocol implementation and error state management, where the lack of proper isolation between authentication attempts creates a pathway for malicious exploitation. The specific configuration requirements of verb 0 logging and absence of TLS authentication create a perfect storm for this vulnerability to manifest, as these settings typically indicate a server environment that prioritizes minimal logging over robust error handling.
The technical flaw directly relates to CWE-248, which addresses improper exception handling in programs, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. The vulnerability demonstrates poor resource management practices within the OpenSSL integration layer, where error state persistence across multiple client sessions creates a condition for service disruption. This issue also reflects broader concerns around secure coding practices in cryptographic applications, particularly the importance of proper error state cleanup and isolation in multi-client environments. The flaw's exploitation requires minimal resources and can be automated, making it particularly dangerous in production environments where OpenVPN servers may be exposed to unauthenticated network traffic. Organizations implementing OpenVPN services without proper TLS authentication and with minimal logging configurations face elevated risk from this vulnerability.
Mitigation strategies for CVE-2005-2531 focus on immediate software updates to OpenVPN 2.0.1 or later versions where the error queue handling has been corrected. Administrators should also implement proper TLS authentication mechanisms to reduce the attack surface, as the vulnerability only manifests when TLS authentication is disabled. Configuration changes including enabling appropriate logging levels and implementing rate limiting for authentication attempts can provide additional defense in depth. Network segmentation and access controls should be implemented to limit exposure of vulnerable OpenVPN servers to untrusted networks. Regular security auditing of VPN configurations should verify that TLS authentication is enabled and that logging levels are appropriately configured to avoid the conditions that trigger this vulnerability. The fix implemented in OpenVPN 2.0.1 specifically addresses the OpenSSL error queue management by ensuring proper cleanup of error states after each authentication attempt, preventing the cross-contamination of error conditions between different client sessions.