When a Multipart request is performed but some of the fields exceed the maxStringLength limit, the upload files will remain in struts.multipart.saveDir even if the request has been denied. Users are recommended to upgrade to versions Struts 2.5.32 or 6.1.2.2 or Struts 6.3.0.1 or greater, which fixe this issue.
https://www.openwall.com/lists/oss-security/2023/12/09/1
https://lists.apache.org/thread/6wj530kh3ono8phr642y9sqkl67ys2ft