Apache Struts 2 before 2.3.14.3 allows remote attackers to execute arbitrary OGNL code via a request with a crafted value that contains both "${}" and "%{}" sequences, which causes the OGNL code to be evaluated twice.
https://cwiki.apache.org/confluence/display/WW/S2-015
http://www.securityfocus.com/bid/64758
http://www.oracle.com/technetwork/topics/security/cpuoct2013-1899837.html
http://www.oracle.com/technetwork/topics/security/cpujan2014-1972949.html