The vulnerability may allow a remote attacker to allocate 100% CPU time on the target system depending on CPU type or parallel execution of such a payload resulting in a denial of service only by manipulating the processed input stream. No user is affected, who followed the recommendation to setup XStream's security framework with a whitelist limited to the minimal required types.
If you rely on XStream's default blacklist of the Security Framework, you will have to use at least version 1.4.16.
See workarounds for the different versions covering all CVEs.
See full information about the nature of the vulnerability and the steps to reproduce it in XStream's documentation for CVE-2021-21341.
The vulnerability was discovered and reported by threedr3am.