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://linux.oracle.com/errata/ELSA-2014-1999.html
http://rhn.redhat.com/errata/RHSA-2014-1999.html
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=278748
http://seclists.org/oss-sec/2014/q4/1066
http://secunia.com/advisories/60940
http://secunia.com/advisories/61585