CVE-2005-2532 in OpenVPN
Summary
by MITRE
OpenVPN before 2.0.1 does not properly flush the OpenSSL error queue when a packet can not be decrypted by the server, which allows remote authenticated attackers to cause a denial of service (client disconnection) via a large number of packets that can not be decrypted.
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Analysis
by VulDB Data Team • 06/11/2019
The vulnerability identified as CVE-2005-2532 affects OpenVPN versions prior to 2.0.1 and represents a significant security flaw in the cryptographic processing pipeline. This issue stems from improper error handling within the OpenSSL library integration, specifically when the server encounters packets that cannot be decrypted during the VPN session. The flaw manifests when the server processes malformed or corrupted packets that fail decryption attempts, creating a scenario where OpenSSL error states accumulate in the error queue without proper clearance.
The technical mechanism behind this vulnerability involves the OpenSSL error queue management within the OpenVPN server implementation. When a packet fails decryption, the system should clear the OpenSSL error queue to prevent state accumulation. However, in affected versions, this cleanup process fails to occur properly, leading to a gradual buildup of error states. This accumulation eventually causes the OpenSSL library to enter a degraded state where it begins to reject legitimate packets or connections, ultimately resulting in client disconnections. The vulnerability is particularly dangerous because it can be exploited through a simple denial of service attack that requires only sending a large number of invalid packets to the server.
The operational impact of this vulnerability extends beyond simple service disruption to create a potential vector for more sophisticated attacks. Remote authenticated attackers can leverage this flaw to repeatedly send malformed packets to the OpenVPN server, causing progressive degradation of the service and eventual client disconnection. This type of attack can be particularly effective in environments where VPN connectivity is critical, as it can disrupt legitimate user access and potentially allow attackers to gain intelligence about the network infrastructure through repeated connection attempts. The vulnerability also demonstrates poor resource management practices in cryptographic applications, where error states are not properly handled, leading to resource exhaustion.
From a cybersecurity perspective, this vulnerability aligns with CWE-399, which addresses Resource Management Errors, and relates to the broader category of denial of service conditions in network security protocols. The flaw also connects to ATT&CK technique T1499.004, which covers network disruption through resource exhaustion, as the accumulation of OpenSSL error states effectively exhausts the server's ability to process legitimate traffic. Organizations should implement immediate mitigation strategies including upgrading to OpenVPN 2.0.1 or later, which includes proper error queue flushing mechanisms. Additionally, network monitoring should be enhanced to detect unusual patterns of client disconnections that may indicate exploitation attempts. The vulnerability underscores the importance of proper error handling in cryptographic libraries and demonstrates how seemingly minor implementation flaws can lead to significant security implications in critical infrastructure components.