The vulnerability tracked as CVE-2025-31647 has been identified in the XStream Java library, a popular tool used for serializing Java objects to XML and back. This flaw arises from insecure deserialization, allowing attackers to send crafted XML payloads that trigger the execution of arbitrary code during the unmarshalling process. What makes this CVE especially critical is that XStream is widely embedded in numerous backend systems, CI/CD pipelines, and automation platforms that consume XML input. Security researchers observed active exploitation in targeted attacks against outdated Jenkins plugins and enterprise automation servers running vulnerable versions. In multiple incidents, threat actors used this vector to deploy reverse shells and establish persistent backdoors in internal networks, bypassing conventional perimeter defenses. The root issue lies in XStream’s permissive deserialization mechanism, which, when unconfigured, permits instantiation of any class on the classpath—including OS-level commands. Mitigations involve upgrading to the latest version where default whitelisting is enforced and disabling deserialization of unknown types altogether. Organizations using XStream in legacy systems should conduct a thorough audit of XML input sources, implement input validation filters, and adopt allowlist strategies to restrict which classes can be deserialized. Given the speed and sophistication of exploitation observed in the wild, immediate patching and detection rule deployment is advised to prevent compromise.