BaseFortify

Publication date: 2025-10-28

Last updated on: 2025-11-07

Assigner: GitHub, Inc.

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.

CVSS Scores

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Meta Information

Published

2025-10-28

Last Modified

2025-11-07

Generated

2026-07-06

AI Q&A

2025-10-28

EPSS Evaluated

2026-07-05

NVD

CVE-2025-62800

Affected Vendors & Products

Vendor	Product	Version / Range
jlowin	fastmcp	to 2.13.0 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-79	The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

Attack-Flow Graph

Executive Summary

This vulnerability is a reflected cross-site scripting (XSS) issue in FastMCP versions prior to 2.13.0. It occurs in the OAuth client callback page (oauth_callback.py) where user-controlled values are inserted into the HTML without proper escaping. This allows an attacker to execute arbitrary JavaScript code in the context of the callback server's origin.

Impact Analysis

An attacker exploiting this vulnerability can execute arbitrary JavaScript in the context of the affected server, potentially leading to session hijacking, theft of sensitive information, or performing actions on behalf of the user without their consent.

Mitigation Strategies

Upgrade FastMCP to version 2.13.0 or later, as this version contains the fix for the reflected cross-site scripting vulnerability in the OAuth client callback page.

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