Check the version of openvpn
Please Install the Updated Packages.
It was discovered that OpenVPN incorrectly handled HMAC comparisons when running in UDP mode. If a remote attacker were able to perform a man-in-the-middle attack, this flaw could possibly be used to perform a plaintext recovery attack.
openvpn on Ubuntu 12.04 LTS
Get the installed version with the help of detect NVT and check if the version is vulnerable or not.