https://struts.apache.org/docs/s2-038.html


Summary
It is possible to bypass token validation and perform a CSRF attack

Who should read this
	

All Struts 2 developers and users

Impact of vulnerability
	

Possible CSRF attack

Maximum security rating
	

Medium

Recommendation
	

Upgrade to Struts 2.3.29.

Affected Software
	

Struts 2.3.20 - Struts Struts 2.3.28.1

Reporter
	

Takeshi Terada websec02 dot g02 at gmail.com

CVE Identifier
	

CVE-2016-4430
Problem

It is possible to pass a malicious expression which can be used to bypass token validation and perform CSRF attack.
Solution

Upgrade to Apache Struts version 2.3.29.
Backward compatibility

Some backward incompatibility issues are expected when upgrading to Struts 2.3.29 - it can happen that some OGNL expressions stop working because of performing disallowed arithmetic operations and assignments.
Workaround

You can try to use more restrictive RegEx used to clean up action names as below:

<constant name="struts.allowed.action.names" value="[a-zA-Z]*" />

Please adjust the RegEx to your action naming pattern, it should be as narrowed as possible.