FastMCP — Xss (CVE-2025-62800)
Date: 2025-10
Affected: jlowin/fastmcp
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2025-62800
CWE: CWE-79
CVSS: 6.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N)

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.


Mitigations:

Architectural layers (MAESTRO):