Jenkins has disclosed a critical remote code execution (RCE) vulnerability, tracked as CVE-2026-70426, affecting its Remoting library used for communication between Jenkins controllers and build agents. The flaw carries a Critical CVSS severity rating and impacts Jenkins 2.575 and earlier, Jenkins LTS 2.568.1 and earlier, and Remoting versions 3384.v60d89463d9e0 and earlier (excluding 3355.3357.v931d3c992987). The issue resides in the Remoting deserialization process, where a security mechanism intended to block unsafe Java classes can be bypassed under specific conditions.

The vulnerability stems from an improper implementation of the JEP-200 class filter, which Jenkins uses to prevent unsafe Java object deserialization during agent-to-controller communication. Researchers discovered that the filter was not enforced when classes were resolved through a fallback deserialization path, allowing attackers to bypass the protection. An attacker who controls a Jenkins build agent, gains code execution on an existing agent, or possesses the Jenkins Agent/Connect permission can exploit this weakness to deserialize restricted Java classes and potentially execute arbitrary code on the Jenkins controller.

Although exploitation is limited to Java classes already present on the Jenkins core classpath or bundled with the Java platform, the impact remains severe because the Jenkins controller is the central management component of the CI/CD environment. Successful exploitation could enable attackers to compromise the controller, exposing sensitive assets such as source code repositories, credentials, deployment keys, secrets, and software supply chain pipelines. While dependencies bundled with Jenkins plugins are not affected through the vulnerable fallback path, a compromised controller still presents a significant risk to enterprise software development and deployment environments.

To address the issue, Jenkins released Jenkins 2.576 and Jenkins LTS 2.568.2, which update the Remoting library to ensure the JEP-200 class filter is consistently enforced, including during fallback deserialization. Organizations are strongly advised to upgrade both Jenkins controllers and agents immediately, review and restrict Agent/Connect permissions, and isolate or closely monitor untrusted build agents to reduce the risk of exploitation. For environments where immediate patching is not feasible, Jenkins has also provided a temporary mitigation through its SECURITY-3911-3930 GitHub repository, although this workaround should only be used until the official security updates can be fully deployed.