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No exploitation reportedAwaiting NVD analysis

CVE-2026-120338

Prefab UI Pyodide sandbox escape to host RCE

No exploitation reported by the sources we read.

  • Published2026-11-01
  • WeaknessCWE-269 · CWE-693 · CWE-94

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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).

Timeline

dates in UTC
  1. 2026-11-0115:24CVE publishedCNA

Observed activity

isMalicious corpus · updated 2026-11-01 15:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-120338?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-120338?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

Products declared by the CNA

As JFROG, the CNA that assigned this CVE, declared them in the CVE record; versions as written. NVD lists no product for this CVE yet: this is the only product data so far.

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-11-01

Neither OSV.dev nor the vendor advisories we read list affected versions for CVE-2026-120338 yet.

Full record on NVD

Fix

No fix published by the sources we read yet. NVD record

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub
Metasploit modules
none listed
Metasploit Framework

Sources

  • CNACVSS 3.1102026-11-01
  • CNAweakness (CWE)CWE-269, CWE-693, CWE-942026-11-01
  • NVDweakness (CWE)CWE-269, CWE-693, CWE-94read 2026-11-01

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Published
2026-11-01
Last Modified
2026-11-01
NVD status
Received

Frequently asked questions

What is CVE-2026-120338?

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).

Is CVE-2026-120338 actively exploited?

Active exploitation of CVE-2026-120338 has not been confirmed.

What is the CVSS score for CVE-2026-120338?

CVE-2026-120338 has a CVSS v3 base score of 10 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H.

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