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.
https://www.tenable.com/blog/one-year-later-what-can-we-learn-from-zerologon
https://www.tenable.com/cyber-exposure/2020-threat-landscape-retrospective
https://www.oracle.com/security-alerts/cpuoct2021.html
https://www.oracle.com/security-alerts/cpujan2021.html
https://www.oracle.com/security-alerts/cpuApr2021.html
https://launchpad.support.sap.com/#/notes/2982840
https://cwiki.apache.org/confluence/display/ww/s2-059
http://packetstormsecurity.com/files/160721/Apache-Struts-2-Forced-Multi-OGNL-Evaluation.html
http://packetstormsecurity.com/files/160108/Apache-Struts-2.5.20-Double-OGNL-Evaluation.html