{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-120338", "assignerOrgId": "48a46f29-ae42-4e1d-90dd-c1676c1e5e6d", "state": "PUBLISHED", "assignerShortName": "JFROG", "dateReserved": "2026-11-01T15:12:50.987Z", "datePublished": "2026-11-01T15:24:54.158Z", "dateUpdated": "2026-11-01T15:24:54.158Z"}, "containers": {"cna": {"providerMetadata": {"orgId": "48a46f29-ae42-4e1d-90dd-c1676c1e5e6d", "shortName": "JFROG", "dateUpdated": "2026-11-01T15:24:54.158Z"}, "title": "Prefab UI Pyodide sandbox escape to host RCE", "problemTypes": [{"descriptions": [{"lang": "en", "cweId": "CWE-693", "description": "CWE-693: Protection Mechanism Failure", "type": "CWE"}]}, {"descriptions": [{"lang": "en", "cweId": "CWE-94", "description": "CWE-94: Improper Control of Generation of Code ('Code Injection')", "type": "CWE"}]}, {"descriptions": [{"lang": "en", "cweId": "CWE-269", "description": "CWE-269: Improper Privilege Management", "type": "CWE"}]}], "affected": [{"vendor": "PrefectHQ", "product": "prefab-ui", "collectionURL": "https://pypi.org/project/prefab-ui/", "packageName": "prefab-ui", "repo": "git://github.com/PrefectHQ/prefab", "programFiles": ["src/prefab_ui/sandbox/_pyodide.py", "src/prefab_ui/sandbox/runner.js"], "versions": [{"status": "affected", "version": "0.14.0", "lessThan": "0.21.0", "versionType": "semver"}], "defaultStatus": "unaffected"}], "descriptions": [{"lang": "en", "value": "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).", "supportingMedia": [{"type": "text/html", "base64": false, "value": "<p>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).</p>"}]}], "references": [{"url": "https://github.com/PrefectHQ/prefab", "name": "Prefab repository", "tags": ["product"]}, {"url": "https://pypi.org/project/prefab-ui/", "name": "prefab-ui on PyPI", "tags": ["product"]}, {"url": "https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9", "name": "GHSA-595q-jv4r-q7j9", "tags": ["vendor-advisory"]}, {"url": "https://github.com/PrefectHQ/prefab/pull/538", "name": "Refresh dependencies and fix renderer and sandbox regressions (fix pull request)", "tags": ["patch"]}, {"url": "https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2", "name": "Fix commit 030f074", "tags": ["patch"]}, {"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0", "name": "v0.21.0", "tags": ["release-notes"]}, {"url": "https://pypi.org/project/prefab-ui/0.21.0/", "name": "prefab-ui 0.21.0", "tags": ["release-notes"]}, {"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2", "name": "v0.20.2 (confirmed affected)", "tags": ["technical-description"]}, {"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0", "name": "v0.14.0 (first release with the sandbox)", "tags": ["technical-description"]}], "metrics": [{"format": "CVSS", "scenarios": [{"lang": "en", "value": "FastMCP server with GenerativeUI enabled, Deno installed, and no authentication configured"}], "cvssV3_1": {"version": "3.1", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH", "baseSeverity": "CRITICAL", "baseScore": 10, "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H"}}, {"format": "CVSS", "scenarios": [{"lang": "en", "value": "Authenticated client allowed to call tools on a server that executes untrusted code in the Generative UI sandbox"}], "cvssV3_1": {"version": "3.1", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH", "baseSeverity": "CRITICAL", "baseScore": 9.9, "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H"}}], "timeline": [{"time": "2026-03-24T17:21:00.000Z", "lang": "en", "value": "Pyodide sandbox added with --allow-read, --allow-write, and --allow-net (commit fc4362f3, pull request 334)"}, {"time": "2026-03-27T02:36:00.000Z", "lang": "en", "value": "prefab-ui 0.14.0 published on PyPI, the first release that includes the sandbox"}, {"time": "2026-06-03T02:13:00.000Z", "lang": "en", "value": "prefab-ui 0.20.2 published. This is the release confirmed in testing."}, {"time": "2026-08-05T16:12:00.000Z", "lang": "en", "value": "Reported to PrefectHQ as GHSA-595q-jv4r-q7j9. The advisory is still in triage and has no CVE id."}, {"time": "2026-10-25T18:48:00.000Z", "lang": "en", "value": "Fix merged in pull request 538 (commit 030f074). The runner revokes read, write, and network permission before accepting code."}, {"time": "2026-10-26T19:00:00.000Z", "lang": "en", "value": "prefab-ui 0.21.0 published on PyPI with the fix. No vendor advisory text was published with the release."}], "credits": [{"lang": "en", "value": "Yuval Moravchick", "type": "finder"}, {"lang": "en", "value": "JFrog Security Research", "type": "coordinator"}], "source": {"discovery": "EXTERNAL"}, "x_generator": {"engine": "Vulnogram 1.0.5"}}}}

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