Happier DOMs: The perils of running untrusted JavaScript code outside of a web browser
Blog post from Endor Labs
Happy-dom, a widely-used headless browser in Node.js, was found vulnerable to remote code execution due to a critical flaw, CVE-2025-61927, allowing malicious JavaScript to escape its sandbox. The package relied on Node.js's vm module, which is not intended for secure execution of untrusted code, enabling attackers to execute arbitrary commands by exploiting weak isolation. Initial mitigation strategies, like avoiding untrusted JavaScript or disabling dynamic code execution primitives, were deemed insufficient as prototype pollution attacks could still occur. This led to a follow-up advisory, CVE-2025-62410, recommending additional security measures such as using the Node.js flag "--frozen-intrinsics" to prevent alteration of built-in objects. Despite these improvements, the risk of pollution remains, prompting suggestions for using more secure sandboxes like isolated-vm or Secure EcmaScript when handling untrusted JavaScript, highlighting the inherent challenges in effectively isolating such code.
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