What is CVE-2025-62800?

CVE-2025-62800 is a medium-severity vulnerability in Jlowin Fastmcp, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2025-10-28.

How severe is CVE-2025-62800?

Medium severity. CVSS v3 base score is 6.1 out of 10.

XSS in Jlowin Fastmcp

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.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2025-10-28 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.003 (18.0th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 18th percentile — 18.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: +0.000

Affected products

Jlowin Fastmcp — versions < 2.13.0

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc (Exploit, Third Party Advisory)

Frequently asked questions

What is CVE-2025-62800?: CVE-2025-62800 is a medium-severity vulnerability in Jlowin Fastmcp, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2025-10-28.
How severe is CVE-2025-62800?: Medium severity. CVSS v3 base score is 6.1 out of 10.