Prefab UI's Generative UI sandbox runs caller-supplied Python inside Pyodide, Python compiled to…

Why it is Amber

• CVSS 10.0 critical
• Remotely exploitable without logging in
• On your watchlist: python

References

• https://github.com/PrefectHQ/prefab
• https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2
• https://github.com/PrefectHQ/prefab/pull/538
• https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0
• https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2
• https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0
• https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9
• https://pypi.org/project/prefab-ui/
• https://pypi.org/project/prefab-ui/0.21.0/

Original description

Prefab UI's Generative UI sandbox runs caller-supplied Python inside Pyodide, Python compiled to WebAssembly, in a Deno subprocess. The feature is documented as isolation for untrusted code, including LLM-written UI. From prefab-ui 0.14.0 through 0.20.2 the launcher starts Deno with unrestricted --allow-read, --allow-write, and --allow-net, and the runner leaves the Deno API on the JavaScript global object. Pyodide's js module proxies that global, so the Python can call Deno.readTextFileSync and Deno.writeTextFileSync. A caller who can supply the code can read and write files as the server OS user, and can persist code execution by writing a .pth file into the server virtualenv's site-packages. The usual remote path is FastMCP's GenerativeUI provider, which registers the MCP tool generate_prefab_ui and forwards that tool's code argument into this sandbox. On a FastMCP server with no authentication configured, any client who can reach the server can call the tool. With authentication, any client allowed to call tools can. Generative UI is opt-in, and the host must have Deno installed. Confirmed on prefab-ui 0.20.2. Fixed in 0.21.0: after the runtime and Prefab sources load, the runner revokes Deno read, write, and network permission before it accepts code, the process is started with --no-prompt so a permission request cannot grant them again, and bootstrap network access is limited to cdn.jsdelivr.net. The fix is commit 030f074 (pull request 538).