The Apache Software Foundation has published a security advisory for Apache Struts 2 Java framework. This vulnerability seems to be widely used in the wild. The problem is described as follows:
Both the s:url and s:a tag provide an includeParams attribute. The main scope of that attribute is to understand whether includes http request parameter or not.
The allowed values of includeParams are:
1. none - include no parameters in the URL (default)
2. get - include only GET parameters in the URL
3. all - include both GET and POST parameters in the URL
A request that included a specially crafted request parameter could be used to inject arbitrary OGNL code into the stack, afterward used as request parameter of an URL or A tag , which will cause a further evaluation.
The second evaluation happens when the URL/A tag tries to resolve every parameters present in the original request. This lets malicious users put arbitrary OGNL statements into any request parameter (not necessarily managed by the code) and have it evaluated as an OGNL expression to enable method execution and execute arbitrary methods, bypassing Struts and OGNL library protections.
This vulnerability is rated highly critical so it's recommended to patch it really quickly. When you try to exploit this vulnerability just after you applied the patch, you still get a shell. Sadly, it seems that the patch doesn't really patch the vulnerability. Exploitation of vulnerabilities can be really efficient to check your risk exposure. The exploit is now available in D2 Elliot: