CVE-2020-15078 in OpenVPN
Summary
by MITRE • 04/26/2021
OpenVPN 2.5.1 and earlier versions allows a remote attackers to bypass authentication and access control channel data on servers configured with deferred authentication, which can be used to potentially trigger further information leaks.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 03/06/2025
OpenVPN version 2.5.1 and earlier implementations contain a critical authentication bypass vulnerability that specifically affects servers configured with deferred authentication mechanisms. This vulnerability stems from improper handling of authentication state during the connection establishment process, creating a window where attackers can manipulate the authentication flow to gain unauthorized access to control channel data. The flaw occurs when the server delays authentication until after the initial connection handshake, leaving a potential attack surface where malicious actors can intercept and manipulate control channel communications. The vulnerability is particularly dangerous because it operates at the protocol level, affecting the fundamental security assumptions of the OpenVPN implementation and potentially enabling attackers to escalate privileges or access sensitive data that should remain protected.
The technical exploitation of this vulnerability involves manipulating the authentication sequence to bypass the normal verification process that occurs after initial connection establishment. Attackers can leverage this weakness to inject malicious control channel data or manipulate existing connections, potentially gaining access to information that should be restricted to authenticated users. The vulnerability specifically targets servers configured with deferred authentication, where authentication occurs after the initial TLS handshake and key exchange phases. This creates a temporal gap in security enforcement that malicious actors can exploit to establish unauthorized connections or modify existing authenticated sessions. The flaw represents a direct violation of the principle of least privilege and can enable attackers to perform unauthorized operations within the VPN infrastructure, potentially leading to complete compromise of the affected systems.
The operational impact of CVE-2020-15078 extends beyond simple unauthorized access to encompass potential information disclosure and privilege escalation within the OpenVPN infrastructure. When exploited, this vulnerability allows attackers to access control channel data that typically contains sensitive configuration information, user credentials, or operational parameters that should remain confidential. The vulnerability can be particularly damaging in enterprise environments where OpenVPN servers manage access to critical internal resources, as it enables attackers to gain insights into network topology, user access patterns, and potentially sensitive operational data. Additionally, the ability to bypass authentication and manipulate control channel communications creates opportunities for further exploitation, including potential lateral movement within the network or the ability to inject malicious commands that could compromise the entire VPN infrastructure.
Organizations affected by this vulnerability should immediately implement mitigations including updating to OpenVPN versions 2.5.2 or later where this issue has been resolved, temporarily disabling deferred authentication mechanisms on affected servers, and implementing additional network segmentation controls to limit the potential impact of successful exploitation. Security teams should also monitor network traffic for unusual control channel communications that might indicate exploitation attempts and conduct thorough vulnerability assessments to identify all affected systems. The vulnerability aligns with CWE-287 which addresses improper authentication issues, and can be categorized under ATT&CK technique T1078 for valid accounts and T1566 for credential harvesting, as it enables unauthorized access to systems through manipulated authentication processes. Organizations should also consider implementing network-based intrusion detection systems to monitor for exploitation attempts and establish incident response procedures specifically tailored to address VPN authentication bypass scenarios.