CVE-2026-120338

10 CRITICALpublic exploit available

Published 2026-11-01 · Updated 2026-11-01

AI risk analysis

Summary: The Generative UI sandbox in prefab-ui incorrectly grants Deno file and network permissions to caller-supplied Python code running inside Pyodide, enabling arbitrary file read/write and persistent code execution via .pth file injection as the host OS user.
Exploitability: Exploitation requires access to a FastMCP server with Generative UI enabled and either no authentication or valid credentials to call tools; Deno must be installed on the host.
Blast radius: An attacker can fully compromise the host by reading sensitive files, writing malicious code into the Python virtual environment, and achieving remote code execution as the server user.
Detection: Look for unexpected file writes (e.g., .pth files) into the Python site-packages directory or Deno API calls (e.g., Deno.readTextFileSync, Deno.writeTextFileSync) from the Pyodide subprocess in process or system logs.
Prioritized remediation: Upgrade to prefab-ui 0.21.0 or later, which revokes Deno permissions before accepting code, starts Deno with --no-prompt, and restricts network access.

rcesandbox-escapefile-injectionwebpythondeno

Analysis generated locally by garnet-coder (no data left the box). AI-assisted — verify against primary sources before acting.

NVD 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).

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Weaknesses

CWE-94, CWE-269, CWE-693

Public exploit & PoC references

All references

Source data: NVD (nvd.nist.gov), public domain. Exploit-DB.ai adds local AI analysis for defensive use only.

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