CVE-2026-120338
CriticalCVSS 10.0Exploitation Probability (EPSS)
Elevated risk56th percentile - higher than 56% of all known CVEs
Summary
The Generative UI sandbox in prefab-ui runs caller-supplied Python inside Pyodide (WebAssembly) in a Deno subprocess, but from 0.14.0 through 0.20.2 Deno starts with unrestricted --allow-read, --allow-write, and --allow-net, and the Deno API remains on the JavaScript global object. This lets Python call Deno.readTextFileSync and Deno.writeTextFileSync, enabling file read/write as the server OS user and persistent code execution via a .pth file in site-packages.
Risk Assessment
A client able to supply code to the generate_prefab_ui tool (e.g., via FastMCP without authentication) can read and write files on the server and persist code execution. This leads to full compromise of the application server.
Recommendation
Update prefab-ui to 0.21.0 (commit 030f074, PR 538), which revokes Deno permissions before accepting code and restricts network access. Until then, do not expose the generate_prefab_ui tool to untrusted clients.
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Original NVD description (English source)
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).
Vulnerability data from NVD (NIST) · CISA KEV · EPSS