CVE-2025-62800 FastMCP vulnerable to reflected XSS in client's callback page
Attackers can execute arbitrary JavaScript in the FastMCP OAuth callback page via reflected XSS, potentially leading to account takeover.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2025-62800?
CVE-2025-62800 is a medium-severity cross-site scripting vulnerability in jlowin fastmcp, published on 28 Oct 2025. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-79
Which versions are affected by CVE-2025-62800?
3 open source packages ship an affected version, including fastmcp. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
fastmcp PyPI | All versions | 2.13.0 |
fastmcp PyPI | All versions | 2.13.0 |
fastmcp PyPI | All versions | 2.13.0 |
Products include Jlowin Fastmcp.
How do I fix CVE-2025-62800?
Upgrade fastmcp to 2.13.0 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2025-62800 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-62800 exists.
Other vulnerabilities in jlowin fastmcp
- CVE-2026-32871 FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability
- CVE-2026-27124 FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities
- CVE-2025-69196 FastMCP OAuth Proxy token reuse across MCP servers
- CVE-2025-64340 FastMCP has a Command Injection vulnerability - Gemini CLI
- CVE-2025-62801 FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name
Sources
Also known as GHSA-mxxr-jv3v-6pgc, PYSEC-2026-1364, ROOT-APP-PYPI-CVE-2025-62800.