= Exploit summary =

OpenVPN 2.3.0 and earlier are subject to a potential timing-based side-channel attack, which is made possible by a non-constant-time HMAC comparison function. Plaintext recovery is possible using a padding oracle attack, optimistically at a rate of about one character per 3 hours. OpenVPN with PolarSSL is vulnerable; the vulnerability of OpenSSL-based OpenVPN has not been verified or tested. 

The [https://github.com/OpenVPN/openvpn/commit/11d21349a4e7e38a025849479b36ace7c2eec2ee fix] for this attack makes the affected function constant-time and thus prevents this exploit.

= Requirements = 

Successful attack requires that

 * OpenVPN is running in UDP mode with a CBC mode cipher
 * The attacker must be able to measure the processing time of the packets

The feasibility of attack is increased significantly if encryption and/or authentication is disabled.
 
= Mitigating factors =

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.

= Affected versions =

OpenVPN 2.3.0 and earlier are vulnerable. A fix ([https://github.com/OpenVPN/openvpn/commit/11d21349a4e7e38a025849479b36ace7c2eec2ee commit f375aa67cc]) is included in OpenVPN 2.3.1 and later.