Prefab UI Pyodide sandbox escape to host RCE

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-120338
StatePUBLISHED
AssignerJFROG
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-11-01 16:18:14 UTC
Updated2026-11-01 16:18:14 UTC
DescriptionPrefab 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).

Risk And Classification

Primary CVSS: v3.1 10 CRITICAL from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Problem Types: CWE-94 | CWE-269 | CWE-693 | CWE-693 CWE-693: Protection Mechanism Failure | CWE-94 CWE-94: Improper Control of Generation of Code ('Code Injection') | CWE-269 CWE-269: Improper Privilege Management

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary10CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
3.1CNACVSS10CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
3.1CNACVSS9.9CRITICALCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA PrefectHQ Prefab-ui affected 0.14.0 0.21.0 semver Not specified

References

ReferenceSourceLinkTags
github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2 [email protected] github.com
github.com/PrefectHQ/prefab/releases/tag/v0.21.0 [email protected] github.com
pypi.org/project/prefab-ui [email protected] pypi.org
github.com/PrefectHQ/prefab/releases/tag/v0.14.0 [email protected] github.com
github.com/PrefectHQ/prefab/releases/tag/v0.20.2 [email protected] github.com
github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9 [email protected] github.com
pypi.org/project/prefab-ui/0.21.0 [email protected] pypi.org
github.com/PrefectHQ/prefab [email protected] github.com
github.com/PrefectHQ/prefab/pull/538 [email protected] github.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Yuval Moravchick (en)

CNA: JFrog Security Research (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-03-24T17:21:00.000ZPyodide sandbox added with --allow-read, --allow-write, and --allow-net (commit fc4362f3, pull request 334)
CNA2026-03-27T02:36:00.000Zprefab-ui 0.14.0 published on PyPI, the first release that includes the sandbox
CNA2026-06-03T02:13:00.000Zprefab-ui 0.20.2 published. This is the release confirmed in testing.
CNA2026-08-05T16:12:00.000ZReported to PrefectHQ as GHSA-595q-jv4r-q7j9. The advisory is still in triage and has no CVE id.
CNA2026-10-25T18:48:00.000ZFix merged in pull request 538 (commit 030f074). The runner revokes read, write, and network permission before accepting code.
CNA2026-10-26T19:00:00.000Zprefab-ui 0.21.0 published on PyPI with the fix. No vendor advisory text was published with the release.

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