FKIE_CVE-2026-120338

Vulnerability from fkie_nvd - Published: 2026-11-01 16:18 - Updated: 2026-11-01 16:18
Severity
10.0 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Summary
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).
References
reefs@jfrog.comhttps://github.com/PrefectHQ/prefab
reefs@jfrog.comhttps://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2
reefs@jfrog.comhttps://github.com/PrefectHQ/prefab/pull/538
reefs@jfrog.comhttps://github.com/PrefectHQ/prefab/releases/tag/v0.14.0
reefs@jfrog.comhttps://github.com/PrefectHQ/prefab/releases/tag/v0.20.2
reefs@jfrog.comhttps://github.com/PrefectHQ/prefab/releases/tag/v0.21.0
reefs@jfrog.comhttps://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9
reefs@jfrog.comhttps://pypi.org/project/prefab-ui/
reefs@jfrog.comhttps://pypi.org/project/prefab-ui/0.21.0/
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "collectionURL": "https://pypi.org/project/prefab-ui/",
          "defaultStatus": "unaffected",
          "packageName": "prefab-ui",
          "product": "prefab-ui",
          "programFiles": [
            "src/prefab_ui/sandbox/_pyodide.py",
            "src/prefab_ui/sandbox/runner.js"
          ],
          "repo": "git://github.com/PrefectHQ/prefab",
          "vendor": "PrefectHQ",
          "versions": [
            {
              "lessThan": "0.21.0",
              "status": "affected",
              "version": "0.14.0",
              "versionType": "semver"
            }
          ]
        }
      ],
      "source": "reefs@jfrog.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Prefab UI\u0027s 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\u0027s 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\u0027s site-packages. The usual remote path is FastMCP\u0027s GenerativeUI provider, which registers the MCP tool generate_prefab_ui and forwards that tool\u0027s 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)."
    }
  ],
  "id": "CVE-2026-120338",
  "lastModified": "2026-11-01T16:18:14.970",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "HIGH",
          "baseScore": 10.0,
          "baseSeverity": "CRITICAL",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "scope": "CHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 6.0,
        "source": "reefs@jfrog.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-11-01T16:18:14.970",
  "references": [
    {
      "source": "reefs@jfrog.com",
      "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/"
    }
  ],
  "sourceIdentifier": "reefs@jfrog.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-94"
        },
        {
          "lang": "en",
          "value": "CWE-269"
        },
        {
          "lang": "en",
          "value": "CWE-693"
        }
      ],
      "source": "reefs@jfrog.com",
      "type": "Primary"
    }
  ]
}



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