The expand function in fio.c in Heirloom mailx 12.5 and earlier and BSD mailx 8.1.2 and earlier allows remote attackers to execute arbitrary commands via shell metacharacters in an email address.
http://www.debian.org/security/2014/dsa-3105
http://secunia.com/advisories/61693
http://secunia.com/advisories/61585
http://secunia.com/advisories/60940
http://seclists.org/oss-sec/2014/q4/1066