Apache Struts 2.0.0 to 2.5.20 forced double OGNL evaluation, when evaluated on raw user input in tag attributes, may lead to remote code execution.
http://packetstormsecurity.com/files/160108/Apache-Struts-2.5.20-Double-OGNL-Evaluation.html
http://packetstormsecurity.com/files/160721/Apache-Struts-2-Forced-Multi-OGNL-Evaluation.html
https://cwiki.apache.org/confluence/display/ww/s2-059
https://launchpad.support.sap.com/#/notes/2982840
https://www.oracle.com/security-alerts/cpuApr2021.html