The ExceptionDelegator component in Apache Struts before 2.2.3.1 interprets parameter values as OGNL expressions during certain exception handling for mismatched data types of properties, which allows remote attackers to execute arbitrary Java code via a crafted parameter.
https://issues.apache.org/jira/browse/WW-3668
http://struts.apache.org/2.x/docs/version-notes-2311.html