Ubuntu Security Notice USN-666-1 – It was discovered that certain email headers were not correctly handled by Dovecot. If a remote attacker sent a specially crafted email to a user with a mailbox managed by Dovecot, that user’s mailbox would become inaccessible through Dovecot, leading to a denial of service.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/71686/USN-666-1.txt
Source: https://packetstormsecurity.com/files/71686/Ubuntu-Security-Notice-666-1.html