An attacker can manipulate file upload params to enable paths traversal and under some circumstances this can lead to uploading a malicious file which can be used to perform Remote Code Execution. Users are recommended to upgrade to versions Struts 2.5.33 or Struts 6.3.0.2 or greater to fix this issue.
https://www.openwall.com/lists/oss-security/2023/12/07/1
https://security.netapp.com/advisory/ntap-20231214-0010/
https://lists.apache.org/thread/yh09b3fkf6vz5d6jdgrlvmg60lfwtqhj
http://packetstormsecurity.com/files/176157/Struts-S2-066-File-Upload-Remote-Code-Execution.html