Prefect Prefab UI up to 0.20.2 Generative UI sandbox

Summaryinfo

A vulnerability has been found in Prefect Prefab UI up to 0.20.2 and classified as very critical. This vulnerability affects unknown code of the component Generative UI. Performing a manipulation results in sandbox. This vulnerability was named CVE-2026-120338. The attack may be initiated remotely. There is no available exploit. The affected component should be upgraded.

Detailsinfo

A vulnerability, which was classified as very critical, was found in Prefect Prefab UI up to 0.20.2. This affects an unknown part of the component Generative UI. The manipulation with an unknown input leads to a sandbox vulnerability. CWE is classifying the issue as CWE-265. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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).

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-120338 since 11/01/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 11/01/2026). The attack technique deployed by this issue is T1611 according to MITRE ATT&CK.

Upgrading to version 0.21.0 eliminates this vulnerability. Applying the patch 030f074 is able to eliminate this problem. The best possible mitigation is suggested to be upgrading to the latest version.

Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Vendor

• Prefect

Name

• Prefab UI

Version

• 0.20.0
• 0.20.1
• 0.20.2

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CPE 2.2info

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Be aware that VulDB is the high quality source for vulnerability data.

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