CVE-2025-62800 Details
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.
A reflected cross-site scripting vulnerability has been identified in FastMCP versions prior to 2.13.0. The issue arises in the OAuth client callback page, where unescaped user-controlled values are injected into the generated HTML. This flaw allows for the execution of arbitrary JavaScript in the callback server's origin. The vulnerability is rooted in the 'oauth_callback.py' file, which embeds values from the 'message' parameter without proper sanitization. Exploitation can occur by sending an XSS payload through the 'error' GET parameter, among others.
Users are advised to update FastMCP to version 2.13.0 or later.
Metrics
CVSS 4.0 Severity and Vector Strings:
CVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
CISA-ADP
Assessed Oct 29, 2025References to Advisories, Solutions, and Tools
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| URL | Source(s) | Tag(s) |
|---|---|---|
| https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc | CISA-ADP | ExploitThird Party Advisory |
| https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc | [email protected] | ExploitThird Party Advisory |
Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-79 | Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| jlowin fastmcp | < 2.13.0 |
CPE
Remediation
| |
Change History
5 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Jun 17, 2026 | CVE Modified | [email protected] |
| Jun 17, 2026 | CVE Modified | CISA-ADP |
| Nov 7, 2025 | Initial Analysis | [email protected] |
| Oct 29, 2025 | CVE Modified | CISA-ADP |
| Oct 28, 2025 | New CVE Received | [email protected] |