XStream is a simple library to serialize objects to XML and back again. In affected versions this 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. XStream 1.4.18 uses no longer a blacklist by default, since it cannot be secured for general purpose.
Use CWE-502, Xstream vendor hub and Xstream product page to widen CVE-2021-39140 into its surrounding weakness, vendor, and product context.
Compare it with CVE-2021-39152, CVE-2021-39150 and CVE-2021-39154 for nearby disclosures in the same product family.