CVE-2025-62800: fastmcp XSS — Exploit

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.

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CVSS v3.1 Score

6.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Weakness Type (CWE)

CWE-79 Cross-site Scripting (XSS)

Affected Products

Vendor	Product	Versions
jlowin	fastmcp	< 2.13.0

References

Exploits

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

Frequently Asked Questions

What is CVE-2025-62800? +

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. It has a CVSS v3.1 base score of 6.1 (MEDIUM).

How severe is CVE-2025-62800? +

CVE-2025-62800 has a CVSS v3.1 score of 6.1 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

What products are affected by CVE-2025-62800? +

CVE-2025-62800 affects products from jlowin, specifically: fastmcp. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-62800? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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CVE-2025-62800