Prefab UI Python Sandbox Flaw Allowed Host File Access; Upgrade to 0.21

A critical vulnerability, CVE-2026-120338, was disclosed in Prefab UI's Generative UI sandbox, affecting versions 0.14.0 through 0.20.2 and rated CVSS 10.0. The sandbox, which runs caller-supplied Python inside Pyodide within a Deno subprocess, was undermined by unrestricted Deno permissions and exposure of the Deno API to Python code. This allowed attackers to read and write files on the host system. Researchers at JFrog Security Research demonstrated that writing a .pth file into a server's virtualenv could execute code outside the sandbox on subsequent interpreter starts. The fixed versions, 0.21.0 and 0.21.1, were released in late October, before the advisory was published, and users of FastMCP with Generative UI enabled are advised to upgrade.