Max Vozeler discovered two format string vulnerabilities in ssmtp, a simple mail transport agent. Untrusted values in the functions die() and log_event() were passed to printf-like functions as format strings. These vulnerabilities could potentially be exploited by a remote mail relay to gain the privileges of the ssmtp process (including potentially root).
For the current stable distribution (woody) this problem will be fixed in version 220.127.116.11.
For the unstable distribution (sid), this problem will be fixed soon.
We recommend that you update your ssmtp package.