SquirrelMail 1.4.11 and 1.4.12, as distributed on sourceforge.net before 20071213, has been externally modified to create a Trojan Horse that introduces a PHP remote file inclusion vulnerability, which allows remote attackers to execute arbitrary code.
http://www.squirrelmail.org/index.php
http://www.securityfocus.com/archive/1/485037/100/0/threaded
http://secunia.com/advisories/28095
http://marc.info/?l=squirrelmail-devel&m=119765235203392&w=2
http://marc.info/?l=squirrelmail-devel&m=119756462212214&w=2