{"id":"CVE-2026-120338","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-120338","generatedAt":"2026-11-04T11:42:57.395Z","title":"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.","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).","published":"2026-11-01T16:18:15.000Z","lastModified":"2026-11-02T20:17:41.000Z","status":"Deferred","sourceIdentifier":"reefs@jfrog.com","cvss":{"version":"3.1","score":10,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H"},"cwe":["CWE-94","CWE-269","CWE-693"],"affected":[],"epss":{"score":0.00771,"percentile":0.55714,"date":"2026-11-03","history":[{"date":"2026-11-02","score":0.00771,"percentile":0.55672}]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"high","text":"High impact if exploited, but EPSS currently rates exploitation as unlikely (0.77%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit."},"changes":[{"kind":"new","label":"New CVE","at":"2026-11-01T16:18:15.000Z","detail":{"score":"10.0","severity":"CRITICAL","publishedAt":"2026-11-01T16:18:14.970Z"},"summary":"New CVE, critical severity (CVSS 10.0)."}],"references":[{"url":"https://github.com/PrefectHQ/prefab","source":"reefs@jfrog.com"},{"url":"https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2","source":"reefs@jfrog.com"},{"url":"https://github.com/PrefectHQ/prefab/pull/538","source":"reefs@jfrog.com"},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0","source":"reefs@jfrog.com"},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2","source":"reefs@jfrog.com"},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0","source":"reefs@jfrog.com"},{"url":"https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9","source":"reefs@jfrog.com"},{"url":"https://pypi.org/project/prefab-ui/","source":"reefs@jfrog.com"},{"url":"https://pypi.org/project/prefab-ui/0.21.0/","source":"reefs@jfrog.com"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-120338","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}