CVE-2025-62800

FastMCP vulnerable to reflected XSS in client's callback page

State: Published
Severity: 5.3 Medium
Published: Oct 28, 2025
Updated: Oct 29, 2025
Reserved: Oct 22, 2025

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.

Scores

CVSS	Score	Vector	Source
4.0	5.3 Medium	CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N	GitHub_M

Weaknesses

CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Affected Products

Product	Versions
fastmcpjlowin	< 2.13.0Affected

References

https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc

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