Remote code execution 0 day in up-to-date Struts 2 applications:

Some months ago Struts2 announced a security vulnerability S2-020 that allowed ClassLoader manipulation and that could be used to get Remote Code Execution on certain application servers like Tomcat 8. The fix for this vulnerability was to forbid the (.*\.|^)class\..* regex from action parameters. However a bypass was made public that basically consists in changing the dot notation for the square bracket notation. So instead of using class.classloader to access the classloader, the bypass used class['classLoader']. I just verified the bypass on my local PoC with latest Struts version (2.3.16.1) and I was able to pop up an evil calc. Also it is possible to bypass the original regex by using Class.classloader (with capital 'C').

Remediation:

While Struts2 releases a fix, please update your excludeParams regex to account for the opening square bracket and capital 'C':

(.*\.|^)(class|Class)(\.|\[).*

Update: After talking with Struts2 security team, they confirmed they are working on the patch and the regex to be released will be:
(.*\.|^|.*|\[('|"))(c|C)lass(\.|('|")]|\[).*

The easiest way is to modify your struts config file and add:

<struts>  
...
...
    <package name="default" namespace="/" extends="struts-default">
        <interceptors>
            <interceptor-stack name="secureParamInterceptor">
                <interceptor-ref name="defaultStack">
                    <param name="params.excludeParams">(.*\.|^|.*|\[('|"))(c|C)lass(\.|('|")]|\[).*,^dojo\..*,^struts\..*,^session\..*,^request\..*,^application\..*,^servlet(Request|Response)\..*,^parameters\..*,^action:.*,^method:.*</param>
                </interceptor-ref>
            </interceptor-stack>
        </interceptors>

        <default-interceptor-ref name="secureParamInterceptor" />
        ...
        ...
    </package>
...
...
</struts>  

Stay secure!