Prefab UI patches CVSS 10.0 sandbox flaw in 0.21
11/02/2026 — 11/02, 07:17·1 sources (all)·1 reports (all)
Key takeaways
- JFrog security research published CVE-2026-120338, a CVSS 10.0 flaw in Prefab UI's generative UI sandbox (versions 0.14.0 to 0.20.2) that le…
- FastMCP users with Generative UI enabled should upgrade first.
Background & analysis
According to CVE-2026-120338, published by JFrog Security Research on November 1, 2026, the generative UI sandbox in Prefab UI versions 0.14.0 through 0.20.2 allowed callers submitting code to read and write files on the host machine. The GitHub advisory lists a CVSS score of 10.0 for the issue.
The fix was released in advance of the disclosure: version 0.21.0 came out on October 26, 2026, and 0.21.1 followed on the same day. The report advises users who have Generative UI enabled in FastMCP to upgrade to the patched releases as a priority.
Prefab UI works by letting an MCP server hand small pieces of generated Python code to a sandbox; the sandbox runs the code and returns a UI, so that the model can render a chart or interface for the user instead of only describing it. In the affected versions, that sandbox boundary could be crossed by the calling party, giving access to host files. The write-up leaves open how widely the vulnerability had been exploited in the wild before the patch, and the current status is simply that 0.21.0 and 0.21.1 are available as the corrected releases.
AI-generated from 1 reports · updated 4 days ago
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JFrog Security Research disclosed CVE-2026-120338 in Prefab UI, rated CVSS 10.0: in versions 0.14.0 through 0.20.2, a caller able to submit code to the Generative UI sandbox could read and write files on the host. The fix shipped in 0.21.0 on October 26, with 0.21.1 following the same day; FastMCP users running Generative UI should upgrade first.
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