CVE-2025-62800: PYSEC-2026-1364 - FastMCP vulnerable to reflected XSS in client's callback page

CVE-2025-62800 · Severity: medium · CVSS 6.1 · Published 2025-10-28

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.

Technologies: fastmcp (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2025-62800 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2025-62800 being exploited in the wild.
How severe is CVE-2025-62800?
Medium severity, with a CVSS score of 6.1, published 28 October 2025.
Which products and versions are affected?
fastmcp (PyPI) ECOSYSTEM:introduced=0,fixed=2.13.0.
Is there a fix for CVE-2025-62800?
Yes. fastmcp (PyPI): upgrade to 2.13.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2025-62800?
Check whether you run Jlowin Fastmcp or fastmcp (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • fastmcp (PyPI): upgrade to 2.13.0 or later
    pip install "fastmcp>=2.13.0"

Affected products

  • Jlowin Fastmcp
  • fastmcp (PyPI) ECOSYSTEM:introduced=0,fixed=2.13.0

Identifiers

  • CVE-2025-62800
  • GHSA-mxxr-jv3v-6pgc
  • PYSEC-2026-1364

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