Apache Struts 2.3.19 to 2.3.20.2, 2.3.21 to 2.3.24.1, and 2.3.25 to 2.3.28, when Dynamic Method Invocation is enabled, allow remote attackers to execute arbitrary code via vectors related to an ! (exclamation mark) operator to the REST Plugin.
https://www.exploit-db.com/exploits/39919/
http://www.securitytracker.com/id/1036017
http://www.securityfocus.com/bid/90960