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Critical Prefab UI vulnerability allowed file access via Python sandbox

By PulseAugur Editorial · [1 sources] ·

A critical vulnerability, CVE-2026-120338, with a CVSS score of 10.0, has been identified in Prefab UI, a generative UI framework from PrefectHQ. The flaw, present in versions 0.14.0 through 0.20.2, allowed attackers to read and write files on the host server by exploiting the Python sandbox environment. While Prefab UI versions 0.21.0 and 0.21.1 have since been released with a fix, users running FastMCP with Generative UI enabled are advised to update immediately. The vulnerability stemmed from unrestricted file system access granted to the Deno subprocess and the exposure of the Deno API through Pyodide's JavaScript global object, effectively bypassing intended isolation. AI

IMPACT This vulnerability highlights the security risks associated with running untrusted code in AI-powered sandboxes, potentially impacting the adoption of generative UI tools.

RANK_REASON Disclosure of a specific CVE affecting a software product.

Read on dev.to — MCP tag →

AI-generated summary · Google Gemini · from 1 sources. How we write summaries →

Critical Prefab UI vulnerability allowed file access via Python sandbox

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COVERAGE [1]

  1. dev.to — MCP tag TIER_1 English(EN) · Gabriel Anhaia ·

    Prefab UI's Python Sandbox Could Read and Write Your Server's Files. Upgrade to 0.21

    <ul> <li> <strong>Book:</strong> <a href="https://www.amazon.com/dp/B0GYMFPTWV" rel="noopener noreferrer">System Design Pocket Guide: Fundamentals</a> </li> <li> <strong>The series:</strong> <em>Pocket Guides for Developers</em> — 8 short books on agents, RAG, prompts, observabil…

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