Critical Remote Code Execution Vulnerability in Happy DOM JavaScript Library

A critical security vulnerability, tracked as CVE-2025-61927, has been discovered in the Happy DOM JavaScript library, which is widely used for server-side rendering and testing frameworks. The flaw allows attackers to escape the virtual machine (VM) context, potentially leading to remote code execution on affected systems. Happy DOM, with over 2.7 million weekly downloads, is integrated into numerous applications, amplifying the potential impact of this vulnerability. The root cause of the issue lies in improper isolation of the Node.js VM context in Happy DOM versions 19 and earlier, which fails to adequately sandbox untrusted code. Security researcher Mas0nShi identified that attackers can exploit the JavaScript constructor inheritance chain to access the global Function constructor, enabling arbitrary code execution. In environments using the CommonJS module system, attackers can further leverage the require() function to import and execute additional modules, broadening the attack surface. While ECMAScript module (ESM) environments restrict some capabilities, they are still affected by the core VM context escape. The vulnerability has been assigned a CVSS score of 9.4, underscoring its severity and the urgency for remediation. Millions of applications that rely on Happy DOM for testing or server-side rendering are at risk if they have not updated to a patched version. The flaw enables attackers to bypass intended security boundaries, potentially compromising the host system and any sensitive data processed within the affected environment. Security advisories recommend immediate updates to the latest version of Happy DOM to mitigate the risk. Organizations are urged to review their software supply chain for dependencies on Happy DOM and to apply patches as soon as possible. The vulnerability highlights the risks associated with improper sandboxing in JavaScript environments, especially in widely adopted open-source libraries. No reports of active exploitation have been confirmed at this time, but the public disclosure and technical details increase the likelihood of exploitation attempts. Security teams should monitor for suspicious activity related to Node.js processes and review application logs for signs of compromise. The incident serves as a reminder of the importance of rigorous security testing and isolation in libraries that execute untrusted code. Developers and DevOps teams should prioritize dependency management and vulnerability scanning to reduce exposure to similar flaws in the future.