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
Upgrade to Struts 2.3.29.
Struts 2.3.20 - Struts Struts 22.214.171.124
Takeshi Terada websec02 dot g02 at gmail.com
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.
Upgrade to Apache Struts version 2.3.29.
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.
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.