A problem has been discovered in the Courier Mail Server. DNS failures were not handled properly when looking up Sender Policy Framework (SPF) records, which could allow attackers to cause memory corruption. The default configuration on Debian has SPF checking disabled, so most machines are not vulnerable. This is explained in the “courier” manpage, section SENDER POLICY FRAMEWORK KEYWORDS.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/39614/dsa-784-1.txt
Source: https://packetstormsecurity.com/files/39614/Debian-Linux-Security-Advisory-784-1.html