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2 vulnerabilities found for prefab-ui by PrefectHQ
CVE-2026-120338 (GCVE-0-2026-120338)
Vulnerability from nvd – Published: 2026-11-01 15:24 – Updated: 2026-11-01 15:24
VLAI
EPSS
VEX
Title
Prefab UI Pyodide sandbox escape to host RCE
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).
Severity
10 (Critical)
9.9 (Critical)
CWE
Assigner
References
9 references
| URL | Tags |
|---|---|
| https://github.com/PrefectHQ/prefab | product |
| https://pypi.org/project/prefab-ui/ | product |
| https://github.com/PrefectHQ/prefab/security/advi… | vendor-advisory |
| https://github.com/PrefectHQ/prefab/pull/538 | patch |
| https://github.com/PrefectHQ/prefab/commit/030f07… | patch |
| https://github.com/PrefectHQ/prefab/releases/tag/… | release-notes |
| https://pypi.org/project/prefab-ui/0.21.0/ | release-notes |
| https://github.com/PrefectHQ/prefab/releases/tag/… | technical-description |
| https://github.com/PrefectHQ/prefab/releases/tag/… | technical-description |
Impacted products
{
"containers": {
"cna": {
"affected": [
{
"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"
}
]
}
],
"credits": [
{
"lang": "en",
"type": "finder",
"value": "Yuval Moravchick"
},
{
"lang": "en",
"type": "coordinator",
"value": "JFrog Security Research"
}
],
"descriptions": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "\u003cp\u003ePrefab 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).\u003c/p\u003e"
}
],
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}
],
"metrics": [
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"attackVector": "NETWORK",
"availabilityImpact": "HIGH",
"baseScore": 10,
"baseSeverity": "CRITICAL",
"confidentialityImpact": "HIGH",
"integrityImpact": "HIGH",
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"scope": "CHANGED",
"userInteraction": "NONE",
"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H",
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"baseScore": 9.9,
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"confidentialityImpact": "HIGH",
"integrityImpact": "HIGH",
"privilegesRequired": "LOW",
"scope": "CHANGED",
"userInteraction": "NONE",
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"version": "3.1"
},
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"scenarios": [
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"descriptions": [
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"cweId": "CWE-94",
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"type": "CWE"
}
]
},
{
"descriptions": [
{
"cweId": "CWE-269",
"description": "CWE-269: Improper Privilege Management",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
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"orgId": "48a46f29-ae42-4e1d-90dd-c1676c1e5e6d",
"shortName": "JFROG"
},
"references": [
{
"name": "Prefab repository",
"tags": [
"product"
],
"url": "https://github.com/PrefectHQ/prefab"
},
{
"name": "prefab-ui on PyPI",
"tags": [
"product"
],
"url": "https://pypi.org/project/prefab-ui/"
},
{
"name": "GHSA-595q-jv4r-q7j9",
"tags": [
"vendor-advisory"
],
"url": "https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9"
},
{
"name": "Refresh dependencies and fix renderer and sandbox regressions (fix pull request)",
"tags": [
"patch"
],
"url": "https://github.com/PrefectHQ/prefab/pull/538"
},
{
"name": "Fix commit 030f074",
"tags": [
"patch"
],
"url": "https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2"
},
{
"name": "v0.21.0",
"tags": [
"release-notes"
],
"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0"
},
{
"name": "prefab-ui 0.21.0",
"tags": [
"release-notes"
],
"url": "https://pypi.org/project/prefab-ui/0.21.0/"
},
{
"name": "v0.20.2 (confirmed affected)",
"tags": [
"technical-description"
],
"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2"
},
{
"name": "v0.14.0 (first release with the sandbox)",
"tags": [
"technical-description"
],
"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0"
}
],
"source": {
"discovery": "EXTERNAL"
},
"timeline": [
{
"lang": "en",
"time": "2026-03-24T17:21:00.000Z",
"value": "Pyodide sandbox added with --allow-read, --allow-write, and --allow-net (commit fc4362f3, pull request 334)"
},
{
"lang": "en",
"time": "2026-03-27T02:36:00.000Z",
"value": "prefab-ui 0.14.0 published on PyPI, the first release that includes the sandbox"
},
{
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"time": "2026-06-03T02:13:00.000Z",
"value": "prefab-ui 0.20.2 published. This is the release confirmed in testing."
},
{
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"time": "2026-08-05T16:12:00.000Z",
"value": "Reported to PrefectHQ as GHSA-595q-jv4r-q7j9. The advisory is still in triage and has no CVE id."
},
{
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"time": "2026-10-25T18:48:00.000Z",
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},
{
"lang": "en",
"time": "2026-10-26T19:00:00.000Z",
"value": "prefab-ui 0.21.0 published on PyPI with the fix. No vendor advisory text was published with the release."
}
],
"title": "Prefab UI Pyodide sandbox escape to host RCE",
"x_generator": {
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}
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"assignerShortName": "JFROG",
"cveId": "CVE-2026-120338",
"datePublished": "2026-11-01T15:24:54.158Z",
"dateReserved": "2026-11-01T15:12:50.987Z",
"dateUpdated": "2026-11-01T15:24:54.158Z",
"state": "PUBLISHED"
},
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}
CVE-2026-120338 (GCVE-0-2026-120338)
Vulnerability from cvelistv5 – Published: 2026-11-01 15:24 – Updated: 2026-11-01 15:24
VLAI
EPSS
VEX
Title
Prefab UI Pyodide sandbox escape to host RCE
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).
Severity
10 (Critical)
9.9 (Critical)
CWE
Assigner
References
9 references
| URL | Tags |
|---|---|
| https://github.com/PrefectHQ/prefab | product |
| https://pypi.org/project/prefab-ui/ | product |
| https://github.com/PrefectHQ/prefab/security/advi… | vendor-advisory |
| https://github.com/PrefectHQ/prefab/pull/538 | patch |
| https://github.com/PrefectHQ/prefab/commit/030f07… | patch |
| https://github.com/PrefectHQ/prefab/releases/tag/… | release-notes |
| https://pypi.org/project/prefab-ui/0.21.0/ | release-notes |
| https://github.com/PrefectHQ/prefab/releases/tag/… | technical-description |
| https://github.com/PrefectHQ/prefab/releases/tag/… | technical-description |
Impacted products
{
"containers": {
"cna": {
"affected": [
{
"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"
}
]
}
],
"credits": [
{
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"type": "finder",
"value": "Yuval Moravchick"
},
{
"lang": "en",
"type": "coordinator",
"value": "JFrog Security Research"
}
],
"descriptions": [
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"lang": "en",
"supportingMedia": [
{
"base64": false,
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"value": "\u003cp\u003ePrefab 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).\u003c/p\u003e"
}
],
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}
],
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"baseScore": 9.9,
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"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0"
},
{
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"tags": [
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"url": "https://pypi.org/project/prefab-ui/0.21.0/"
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{
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"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2"
},
{
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"tags": [
"technical-description"
],
"url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0"
}
],
"source": {
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},
"timeline": [
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},
{
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"time": "2026-03-27T02:36:00.000Z",
"value": "prefab-ui 0.14.0 published on PyPI, the first release that includes the sandbox"
},
{
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"time": "2026-06-03T02:13:00.000Z",
"value": "prefab-ui 0.20.2 published. This is the release confirmed in testing."
},
{
"lang": "en",
"time": "2026-08-05T16:12:00.000Z",
"value": "Reported to PrefectHQ as GHSA-595q-jv4r-q7j9. The advisory is still in triage and has no CVE id."
},
{
"lang": "en",
"time": "2026-10-25T18:48:00.000Z",
"value": "Fix merged in pull request 538 (commit 030f074). The runner revokes read, write, and network permission before accepting code."
},
{
"lang": "en",
"time": "2026-10-26T19:00:00.000Z",
"value": "prefab-ui 0.21.0 published on PyPI with the fix. No vendor advisory text was published with the release."
}
],
"title": "Prefab UI Pyodide sandbox escape to host RCE",
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