VULNpocalypse

CVE-2026-120338: Prefab UI Pyodide sandbox escape to host RCE

Advisory generated from the CVE record, CISA, Exploit Observer and VEDAS: what it is, whether it is exploited, who is affected, and how to fix and detect it.

Generated by VEDAS. Scores as of 2026-11-08 00:03 UTC. Rebuilt every few hours from cve-scores, CISA KEV and vedas-signatures. How this works and its limits.

What it 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).

Description quoted from the CVE record (assigned by JFROG, published 2026-11-01).

Weakness: CWE-693: Protection Mechanism Failure; CWE-94: Improper Control of Generation of Code (‘Code Injection’); CWE-269: Improper Privilege Management

How it can be exploited

Exploitable over the network with low attack complexity needing no privileges and no user interaction. Impact: 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

CISA assessment (SSVC): CISA has no evidence of exploitation. Automatable: no. Technical impact: total.

Public exploit evidence (Exploit Observer)

Exploit Observer found 3 code repositories naming this CVE.

Code repositories that name a CVE are often scanners, copies or fakes, and some are malware. Read the code before running anything.

See all 5 references on Exploit Observer

Live scores

EPSS
0.008
low predicted chance of attack
VEDAS
0.986
strong exploit evidence
On CISA KEV
Not listed
Detection rule
Suricata

Who is affected

ProductAffectedNot affected
PrefectHQ prefab-uifrom 0.14.0 before 0.21.0Not stated

Version ranges exactly as the CVE record states them. “Before X” means X is the first fixed version.

How to fix it

Fixed versions stated in the CVE record: 0.21.0.

Vendor guidance:

How to detect it

Suricata rules (network detection)

1 rule that alert on traffic matching this exploit: VEDAS CVE-2026-120338 Prefab UI Generative UI Sandbox Escape - Remote Code Execution. Add them to your Suricata rule set and reload.

Show the rules
suricata
alert http $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS (msg:"VEDAS CVE-2026-120338 Prefab UI Generative UI Sandbox Escape - Remote Code Execution"; flow:established,to_server; http.method; content:"POST"; http.request_body; content:"generate_prefab_ui"; nocase; fast_pattern; content:"Deno"; nocase; pcre:"/Deno\s*\.\s*(?:readTextFileSync|writeTextFileSync|readFileSync|writeFileSync|writeTextFile|readTextFile|Command|run)/"; classtype:web-application-attack; reference:cve,CVE-2026-120338; reference:url,https://www.exploit.observer/?keyword=CVE-2026-120338&match=exact; reference:url,https://subdomain.center/?engine=ammonites&keyword=prefab-ui&match=exact; sid:1102803; rev:1;)

CVE-2026-120338.rules on GitHub

References

Vendor advisories and patches

Technical analysis and advisories

Other references

Other databases

Where this comes from

This advisory is generated automatically by VEDAS from: CVE.org record, cve-scores (EPSS and VEDAS), Exploit Observer, vedas-signatures. Text in quotes is reproduced from the source; everything else is a fixed template filled with source fields. Nothing here is written by a language model. Check the vendor advisory before acting, and read how VEDAS and Exploit Observer work, and their limits.

Supporting partners

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