Sandbox Misconfiguration in Prefab UI by PrefectHQ
CVE-2026-120338

9.9CRITICAL

Key Information:

Vendor

Prefecthq

Status
Prefab-ui
Vendor
CVE Published:
1 November 2026

What is CVE-2026-120338?

The Prefab UI application by PrefectHQ features a Generative UI sandbox intended to carefully isolate untrusted code execution. However, from versions 0.14.0 to 0.20.2, this sandbox runs Python code supplied by users in a Deno subprocess with excessive permissions, enabling unauthorized file access and potential code execution. This misconfiguration allows attackers to read and write files as the server operating system user, and even persist malicious code by writing to the server's virtual environment. Unauthenticated users can exploit this vulnerability when able to access a FastMCP server that uses the GenerativeUI provider. The issue has been addressed in version 0.21.0 where permissions have been escalated and restricted to prevent such exploits.

Affected Version(s)

prefab-ui 0.14.0 < 0.21.0

References

https://github.com/PrefectHQ/prefab
product
https://pypi.org/project/prefab-ui/
product
https://github.com/PrefectHQ/prefab/security/advisories/G...
vendor-advisory

CVSS V3.1

Score:
9.9
Severity:
CRITICAL
Confidentiality:
High
Integrity:
High
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
None
Scope:
Changed

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Yuval Moravchick
JFrog Security Research
.