In Struts 2 before 2.3.15.1 the information following "action:", "redirect:", or "redirectAction:" is not properly sanitized and will be evaluated as an OGNL expression against the value stack. This introduces the possibility to inject server side code.
id: CVE-2013-2251
info:
name: Apache Struts 2 - DefaultActionMapper Prefixes OGNL Code Execution
...