Prefab UI: Arbitrary code execution via Generative UI sandbox (CVE-2026-120338)

The Prefab UI Generative UI feature utilizes a Deno subprocess to execute Pyodide-based Python code. In versions 0.14.0 through 0.20.2, this subprocess is initiated with unrestricted --allow-read, --allow-write, and --allow-net permissions. Because the Deno API remains available on the JavaScript global object, the Python sandbox can invoke privileged functions such as Deno.readTextFileSync and Deno.writeTextFileSync. An attacker providing code through the generate_prefab_ui tool can read/write files as the server OS user or achieve code execution persistence by creating .pth files in the server's virtual environment.