Ubuntu Security Notice USN-678-1 – 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/72311/USN-678-1.txt
Source: https://packetstormsecurity.com/files/72311/Ubuntu-Security-Notice-678-1.html