Prefab UI's Generative UI sandbox has been identified as vulnerable to remote code execution (RCE) via a Pyodide js Bridge to Deno. This vulnerability, tracked as CVE-2026-120338, has been assigned a CVSS score of 10.0, indicating critical severity.
The vulnerability affects Prefab UI versions 0.14.0 to 0.20.2 and allows attackers to execute arbitrary Python code by exploiting Deno's permissions. This includes file read/write access on the host server.
The usual remote path for exploitation is through FastMCP's GenerativeUI provider, particularly on servers with no authentication configured, where any client who can reach the server can call the vulnerable tool.
A patch addressing this vulnerability has been released in version 0.21.0 of Prefab UI. The vulnerability was publicly disclosed on November 1, 2026.
Currently, there is no confirmed public exploit code for CVE-2026-120338.