https://struts.apache.org/docs/s2-040.html

Input validation bypass using existing default action method.

Who should read this
	

All Struts 2 developers and users

Impact of vulnerability
	

Possible manipulation of return result and bypassing validation

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-2016-4431
Problem

Using existing default method it can be possible to bypass internal security mechanism and manipulate return string which can leads to redirecting user to unvalidated location.
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.