{"id":"CVE-2026-120338","published":"2026-11-01T16:18:14.970","lastModified":"2026-11-01T16:18:14.970","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).","cvssScore":10,"cvssSeverity":"CRITICAL","cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H","cwes":["CWE-94","CWE-269","CWE-693"],"vendors":[],"products":[],"references":[{"url":"https://github.com/PrefectHQ/prefab","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/pull/538","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9","tags":[]},{"url":"https://pypi.org/project/prefab-ui/","tags":[]},{"url":"https://pypi.org/project/prefab-ui/0.21.0/","tags":[]}],"exploitRefs":[{"url":"https://github.com/PrefectHQ/prefab","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/pull/538","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0","tags":[]},{"url":"https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9","tags":[]}],"hasPoc":true,"ai":{"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.","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.","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.","tags":["rce","sandbox-escape","file-injection","web","python","deno"],"model":"garnet-coder","analyzedAt":"2026-11-02T09:45:06.567Z"}}