CVE-2026-120338: Prefab UI's Generative UI sandbox runs caller-supplied Python inside Pyodide, Python compiled to WebAssembly,

This month

CRITICAL 10UPGRADE

CVE-2026-120338 is a critical-severity flaw (CVSS 10) affecting prefab-ui. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

Critical severity

A fixed release or distro patch exists.

Risk facts

CVSS score	10 (Critical)
EPSS exploit chance	n/a
CISA exploited list	No
Weakness	CWE-693, CWE-94, CWE-269
Published	2026-11-01
Last modified	2026-11-01

Publisher (CNA): JFROG · Vendor: prefecthq · Source: CVE Program record (NVD analysis pending)

Last updated 2026-11-01 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-11-01 15:24 UTCPublished CVE record published
2026-11-01 15:24 UTCUpdated Record last modified

prefecthq prefab-ui UPGRADE

Affected: >= 0.14.0 and < 0.21.0

Fixed in: 0.21.0

Safe to downgrade to: a release before 0.14.0

1. Check installed version

apt-cache policy prefab-ui || rpm -q prefab-ui || prefab-ui --version

2. UPGRADE to 0.21.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade prefab-ui

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade prefab-ui

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 0.14.0 (Debian / Ubuntu)

apt-cache madison prefab-ui sudo apt install prefab-ui=<VERSION_BEFORE_0.14.0> sudo apt-mark hold prefab-ui

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list prefab-ui sudo dnf downgrade prefab-ui-<VERSION_BEFORE_0.14.0> sudo dnf versionlock add prefab-ui

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Advisories and coverage

Research and news

VulDB Recent Entries: CVE-2026-120338 | Prefect Prefab UI up to 0.20.2 Generative UI sandbox (030f074) 2026-11-01

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-120338?

prefab-ui: upgrade to 0.21.0. Exact commands for each OS are on this page.

Is CVE-2026-120338 being exploited?

CVE-2026-120338 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-120338 affect?

CVE-2026-120338 affects prefab-ui. Severity is critical (CVSS 10). 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 t

How urgent is CVE-2026-120338?

Priority 35/100. Critical severity

Software pages

Python vulnerabilities

References

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