CVE-2020-36382 in Access Server
Summary
by MITRE • 06/04/2021
OpenVPN Access Server 2.7.3 to 2.8.7 allows remote attackers to trigger an assert during the user authentication phase via incorrect authentication token data in an early phase of the user authentication resulting in a denial of service.
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Analysis
by VulDB Data Team • 06/07/2021
The vulnerability identified as CVE-2020-36382 affects OpenVPN Access Server versions 2.7.3 through 2.8.7, representing a critical denial of service weakness that can be exploited by remote attackers. This flaw manifests during the user authentication phase when malicious actors submit incorrect authentication token data during an early stage of the authentication process. The vulnerability stems from insufficient input validation and error handling mechanisms within the authentication subsystem, allowing attackers to manipulate the authentication flow in a manner that triggers an internal assert condition.
The technical implementation of this vulnerability involves the server's failure to properly validate authentication token data during the initial authentication handshake. When malformed or incorrect token data is submitted, the system's assertion mechanism is activated, causing the service to terminate abruptly and resulting in a denial of service condition. This represents a classic example of inadequate error handling where the system does not gracefully manage invalid inputs but instead fails catastrophically. The vulnerability aligns with CWE-248, which addresses "Uncaught Exception," and demonstrates how improper exception handling can lead to system instability and service disruption.
From an operational perspective, this vulnerability poses significant risk to organizations relying on OpenVPN Access Server for remote access services. Attackers can repeatedly exploit this weakness to disrupt legitimate user access, potentially causing business disruption and productivity loss. The remote nature of the attack means that threat actors do not require physical access or network proximity to exploit the vulnerability, making it particularly dangerous in environments where network segmentation is not properly implemented. The attack can be executed with minimal resources and technical expertise, as it does not require advanced privileges or specialized tools beyond basic network connectivity.
The impact extends beyond simple service disruption to include potential operational costs associated with incident response and system recovery. Organizations may experience extended downtime while administrators investigate and remediate the issue, particularly if the vulnerability is exploited repeatedly or in a coordinated attack. The vulnerability also creates opportunities for attackers to perform reconnaissance activities, as successful exploitation can provide information about the system's internal state and authentication mechanisms. Security teams must consider this vulnerability in their threat modeling and incident response planning, as it represents a straightforward method for conducting denial of service attacks against critical network infrastructure.
Mitigation strategies should include immediate patching of affected OpenVPN Access Server versions to the latest releases that contain the necessary fixes. Organizations should also implement network monitoring to detect unusual authentication patterns that may indicate exploitation attempts. The implementation of rate limiting and authentication attempt throttling can help reduce the effectiveness of automated exploitation attempts. Additionally, security teams should consider network segmentation to limit the potential impact of successful exploitation and ensure that authentication systems are properly hardened against malformed input. This vulnerability highlights the importance of robust input validation and proper error handling in network security applications, as outlined in various security frameworks including the OWASP Top Ten and NIST cybersecurity guidelines.