S2-012

Summary

Showcase app 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 a property, afterward used as request parameter of a redirect address, which will cause a further evaluation.

OGNL evaluation was already addressed in S2-003 and S2-005 and S2-009, but, since it involved just the parameter's name, it turned out that the resulting fixes based on whitelisting acceptable parameter names and denying evaluation of the expression contained in parameter names, closed the vulnerability only partially.

The second evaluation happens when redirect result reads it from the stack and uses as a the previously injected code as redirect parameter.
This lets malicious users put arbitrary OGNL statements into any String variable exposed by an action and have it evaluated as an OGNL expression to disable method execution and execute arbitrary methods, bypassing Struts and OGNL library protections.

Proof of concept

1. Run struts2-showcase
2. Open url: http://localhost:8080/struts2-showcase/skill/edit.action?skillName=SPRING-DEV
3. write skill name to %
   Unknown macro: {expr}
   for example:

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

4. submit the form

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 regex pattern inside the ParameterInterceptor was changed to provide a more narrow space of acceptable parameter names.
Furthermore the new setParameter method provided by the value stack will allow no more eval expression inside the param names.

In case an upgrade isn't possible in a particular environment, there is a configuration based mitigation workaround:

Possible Mitigation Workaround: Configure ParametersIntercptor in struts.xml to Exclude Malicious Parameters

The following additional interceptor-ref configuration should mitigate the problem when applied correctly, allowing only simple navigational expression:

<interceptor-ref name="params">
	<param name="acceptParamNames">\w+((\.\w+)|(\[\d+\])|(\['\w+'\]))*</param>
</interceptor-ref>