OpenVPN Access Server older than version 2.8.4 and version 2.9.5 generates new user authentication tokens instead of reusing exiting tokens on reconnect making it possible to circumvent the initial token expiry timestamp.

An issue was discovered in OpenVPN Access Server before 2.7.0 and 2.8.x before 2.8.3. With the full featured RPC2 interface enabled, it is possible to achieve a temporary DoS state of the management interface when sending an XML Entity Expansion (XEE) payload to the XMLRPC based RPC2 interface. The duration of the DoS state depends on available memory and CPU speed. The default restricted mode of the RPC2 interface is NOT vulnerable.

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.

OpenVPN Access Server 2.10 and prior versions are susceptible to resending multiple packets in a response to a reset packet sent from the client which the client again does not respond to, resulting in a limited amplification attack.

OpenVPN Access Server before 2.11 uses a weak random generator used to create user session token for the web portal

The OpenVPN Access Server installer creates a log file readable for everyone, which from version 2.10.0 and before 2.11.0 may contain a random generated admin password

Using the --fragment option in certain configuration setups OpenVPN version 2.6.0 to 2.6.6 allows an attacker to trigger a divide by zero behaviour which could cause an application crash, leading to a denial of service.

OpenVPN 2.x before 2.0.11, 2.1.x, 2.2.x before 2.2.3, and 2.3.x before 2.3.6 allows remote authenticated users to cause a denial of service (server crash) via a small control channel packet.

Multiple cross-site request forgery (CSRF) vulnerabilities in the XML-RPC API in the Desktop Client in OpenVPN Access Server 1.5.6 and earlier allow remote attackers to hijack the authentication of administrators for requests that (1) disconnecting established VPN sessions, (2) connect to arbitrary VPN servers, or (3) create VPN profiles and execute arbitrary commands via crafted API requests.

OpenVPN Access Server 2.8.7 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.

According to the upstream security report [1] the following flaw was found in OpenVPN 2.3.0 and earlier:

OpenVPN 2.3.0 and earlier running in UDP mode are subject to chosen ciphertext injection due to a non-constant-time HMAC comparison function. Plaintext recovery may be possible using a padding oracle attack on the CBC mode cipher implementation of the crypto library, optimistically at a rate of about one character per 3 hours. PolarSSL seems vulnerable to such an attack; the vulnerability of OpenSSL has not been verified or tested.

OpenVPN servers are typically configured to silently drop packets with the wrong HMAC. For this reason measuring the processing time of the packets is not trivial without a MITM position. In practice, the attack likely needs some target-specific information to be effective.

The severity of this vulnerability can be considered low. Only if OpenVPN is configured to use a null-cipher, arbitrary plain-text can be injected which can completely open up this attack vector.

A fix has been committed to git [2]; this is corrected in OpenVPN 2.3.1 and later.

[1] https://community.openvpn.net/openvpn/wiki/SecurityAnnouncement-f375aa67cc [2] https://github.com/OpenVPN/openvpn/commit/11d21349a4e7e38a025849479b36ace7c2eec2ee