{
  "id": "CVE-2026-120338",
  "url": "https://spydr.io/cve/CVE-2026-120338",
  "published": "2026-11-01T16:18:14.970Z",
  "modified": "2026-11-01T16:18:14.970Z",
  "score": 10,
  "severity": "critical",
  "cvss_version": "3.1",
  "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H",
  "score_source": "jfrog.com",
  "epss": null,
  "epss_percentile": null,
  "exploited": false,
  "kev": null,
  "ssvc_exploitation": null,
  "vendors": [
    "PrefectHQ"
  ],
  "products": [
    "PrefectHQ prefab-ui"
  ],
  "cwes": [
    "CWE-94",
    "CWE-269",
    "CWE-693"
  ],
  "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).",
  "status": "Received",
  "score_type": "Secondary",
  "scores": {
    "cvss_v40": null,
    "cvss_v31": 10,
    "cvss_v30": null
  },
  "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": []
    }
  ],
  "nvd_url": "https://nvd.nist.gov/vuln/detail/CVE-2026-120338",
  "covered_in": [],
  "attribution": [
    {
      "source": "NVD",
      "url": "https://nvd.nist.gov",
      "notice": "This product uses data from the NVD API but is not endorsed or certified by the NVD."
    },
    {
      "source": "CISA KEV",
      "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog",
      "notice": "Known exploited vulnerabilities from the CISA KEV catalog."
    },
    {
      "source": "FIRST EPSS",
      "url": "https://www.first.org/epss",
      "notice": "Exploit prediction scores from FIRST EPSS."
    }
  ]
}