# CVE-2026-120338: CVE-2026-120338

Status: published. Published: 2026-11-01. Updated: 2026-11-01.

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).

## Remediation

No remediation reported in collected sources

## Affected products and versions

Not reported in collected sources.

## Severity and prioritization

- CVSS 3.1: 10 (critical); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H; source: NVD: reefs@jfrog.com

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-94

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/PrefectHQ/prefab (reefs@jfrog.com)

- https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2 (reefs@jfrog.com)

- https://github.com/PrefectHQ/prefab/pull/538 (reefs@jfrog.com)

- https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0 (reefs@jfrog.com)

- https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2 (reefs@jfrog.com)

- https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0 (reefs@jfrog.com)

- https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9 (reefs@jfrog.com)

- https://pypi.org/project/prefab-ui/ (reefs@jfrog.com)

- https://pypi.org/project/prefab-ui/0.21.0/ (reefs@jfrog.com)

## Provenance

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-120338; retrieved 2026-11-02; source updated 2026-11-01

Revision: dad338d023099c30288dcc0295dea21bedfd97ecee14077daff0f7553fef45a3

Canonical: https://y2.dev/cve/CVE-2026-120338