Summary

Problem

OGNL provides, among other features, extensive expression evaluation capabilities. The vulnerability allows a malicious user to bypass all the protections (regex pattern, deny method invocation) built into the ParametersInterceptor, thus being able to inject a malicious expression in any exposed string variable for further evaluation.

A similar behavior was already addressed in S2-003 and S2-005, but it turned out that the resulting fix based on whitelisting acceptable parameter names closed the vulnerability only partially.
According to the regex pattern present into the ParametersInterceptor top'foo'(0) is a valid expression, unfortunately expression like this are further evaluated, hence resolved, by the OGNL parser. Then, a malicious user could inject into the foo var one of the following reserved keyword, bypassing the beforementioned filter:

• #context - OgnlContext, the one guarding method execution based on 'xwork.MethodAccessor.denyMethodExecution' property value.
• #_memberAccess - SecurityMemberAccess, whose 'allowStaticAccess' field prevented static method execution.
• #root
• #this
• #_typeResolver
• #_classResolver
• #_traceEvaluations
• #_lastEvaluation
• #_keepLastEvaluation

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, suggested by John Wilander, should mitigate the problem when applied correctly:

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