---
id: "CVE-2026-120338"
kind: "vulnerability"
aliases: ["CVE-2026-120338"]
title: "Prefab UI: Generative UI sandbox lets Python read and write server files"
severity: "critical"
cvss: 10.0
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["fastmcp", "mcp", "package", "prefab-ui", "python", "self-hosted"]
not_affected_if: ["generative-ui-disabled", "no-deno-installed", "version-0.21.0-or-later"]
published_at: "2026-11-01T16:18:14+00:00"
first_seen_at: "2026-11-01T16:20:45+00:00"
updated_at: "2026-11-01T16:22:42+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-120338", "first_seen_at": "2026-11-01T16:20:45+00:00"}]
---

# Prefab UI: Generative UI sandbox lets Python read and write server files

Prefab UI's Generative UI sandbox exposes the Deno API to caller-supplied Python code, so the code can read and write files as the server OS user. An attacker can also persist code execution by writing a .pth file into the server virtualenv's site-packages.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed version of `prefab-ui` in the server virtualenv, for example with `pip show prefab-ui`.
- Check whether FastMCP's GenerativeUI provider is enabled and whether the `generate_prefab_ui` tool is registered.
- Check whether `deno` is installed on the host.
- Check whether the FastMCP server has authentication configured.

## Fix

- `prefab-ui` `0.21.0` (fixed; affected range 0.14.0 through 0.20.2)

## Timeline

- 2026-11-01T16:20:45+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-120338

## From the sources

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).

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_