Zabbix before 1.8.19rc1, 2.0 before 2.0.10rc1, and 2.2 before 2.2.1rc1 allows remote Zabbix servers and proxies to execute arbitrary commands via a newline in a flexible user parameter.
http://www.zabbix.com/rn2.2.1rc1.php
http://www.zabbix.com/rn2.0.10rc1.php