CVE-2025-62800

FastMCP vulnerable to reflected XSS in client's callback page

Published Oct 28, 2025 CVSS 4.0 · 5.3 Medium EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-mxxr-jv3v-6pgc FastMCP vulnerable to reflected XSS in client's callback page
History
Type Values Removed Values Added
First Time appeared Jlowin
Jlowin fastmcp
CPEs cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*
Vendors & Products Jlowin
Jlowin fastmcp
Metrics cvssV3_1

{'score': 6.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N'}

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Fastmcp
Fastmcp fastmcp
Vendors & Products Fastmcp
Fastmcp fastmcp
Type Values Removed Values Added
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.
Title FastMCP vulnerable to reflected XSS in client's callback page
Weaknesses CWE-79
References
Metrics cvssV4_0

{'score': 5.3, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Oct 29, 2025, 3:52 PM
Reserved
Oct 22, 2025, 6:55 PM
Status
Analyzed
Published
Oct 28, 2025, 10:15 PM
Modified
Oct 8, 2026, 11:10 AM