Ubuntu Security Notice USN-678-2 – USN-678-1 fixed a vulnerability in GnuTLS. The upstream patch introduced a regression when validating certain certificate chains that would report valid certificates as untrusted. This update fixes the problem. We apologize for the inconvenience. Original advisory details: Martin von Gagern discovered that GnuTLS did not properly verify certificate chains when the last certificate in the chain was self-signed. If a remote attacker were able to perform a man-in-the-middle attack, this flaw could be exploited to view sensitive information.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/72832/USN-678-2.txt
Source: https://packetstormsecurity.com/files/72832/Ubuntu-Security-Notice-678-2.html