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CVE-2025-62800

Medium

Description

FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.

SeverityMedium
CVSS6.1
Exploit probability—
ProductJlowin Fastmcp
PublishedOct 28, 2025
First seenOct 28, 2025
Weakness typeCWE-79Cross-site scripting (XSS)

How to fix

Update
  • jlowin fastmcp2.13.0 or later
  • fastmcp2.13.0 or later

Affected products

ProductAffected versionsFixed in
jlowin fastmcpv < 2.13.02.13.0
fastmcpPyPIv < 2.13.02.13.0

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