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

Source: https://junglewise.ai/threats/cve-2025-62800-fastmcp-vulnerable-to-reflected-xss-in-client-s-callback-page  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-10-28  
License: CC BY 4.0 with a link back

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)](https://junglewise.ai/threats/technologies/fastmcp). Vendors: [PyPI](https://junglewise.ai/threats/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

  ```sh
  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

## Related threats

- [FastMCP post-transform component authorization bypass](https://junglewise.ai/threats/fastmcp-post-transform-component-authorization-is-skipped-on-direct-a9992939)
- [FastMCP OAuth provider missing issuer binding in M2M credential exchange](https://junglewise.ai/threats/fastmcp-mcp-client-wrappers-construct-m2m-oauth-providers-without-af21ca1f)
- [FastMCP confused deputy vulnerability in OAuthProxy callback](https://junglewise.ai/threats/cve-2026-27124-fastmcp-confused-deputy-vulnerability-in-oauthproxy-callback)
- [PrefectHQ FastMCP command injection in CLI install on Windows](https://junglewise.ai/threats/cve-2025-64340-prefecthq-fastmcp-command-injection-in-cli-install-on-windows)
- [PrefectHQ FastMCP path traversal and SSRF in OpenAPIProvider](https://junglewise.ai/threats/cve-2026-32871-prefecthq-fastmcp-path-traversal-and-ssrf-in-openapiprovider)