Headline
CVE-2021-31805: S2-062 - Apache Struts 2 Wiki
The fix issued for CVE-2020-17530 was incomplete. So from Apache Struts 2.0.0 to 2.5.29, still some of the tag’s attributes could perform a double evaluation if a developer applied forced OGNL evaluation by using the %{…} syntax. Using forced OGNL evaluation on untrusted user input can lead to a Remote Code Execution and security degradation.
Summary
Forced OGNL evaluation, when evaluated on raw not validated user input in tag attributes, may lead to remote code execution - same as S2-061.
Who should read this
All Struts 2 developers and users
Impact of vulnerability
Possible Remote Code Execution vulnerability
Maximum security rating
Important
Recommendation
Upgrade to Struts 2.5.30 or greater
Affected Software
Struts 2.0.0 - Struts 2.5.29
Reporters
Chris McCown
CVE Identifier
CVE-2021-31805
Problem
The fix issued for CVE-2020-17530 (S2-061) was incomplete. Still some of the tag’s attributes could perform a double evaluation if a developer applied forced OGNL evaluation by using the %{...} syntax. Using forced OGNL evaluation on untrusted user input can lead to a Remote Code Execution and security degradation.
Solution
Avoid using forced OGNL evaluation on untrusted user input, and/or upgrade to Struts 2.5.30 or greater which checks if expression evaluation won’t lead to the double evaluation.
DISCLAIMER
Struts won’t accept double evaluation issues caused by not validated end-user input (owing to developer error) anymore as vulnerability. We accepted this one as vulnerability because it’s about an error in our previously accepted vulnerability. We welcome and appreciate reports in this regard to minimize developer error effect albeit!
Backward compatibility
No issues expected when upgrading to Struts 2.5.30
Workaround
Do not use forced OGNL evaluation in the tag’s attributes based on untrusted/unvalidated user input, please follow out recommendations from the Security Guide.