Moderate severityNVD Advisory· Published Oct 28, 2025· Updated Oct 29, 2025
FastMCP vulnerable to reflected XSS in client's callback page
CVE-2025-62800
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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Affected packages
Versions sourced from the GitHub Security Advisory.
| Package | Affected versions | Patched versions |
|---|---|---|
fastmcpPyPI | < 2.13.0 | 2.13.0 |
Affected products
2Patches
Vulnerability mechanics
References
5- github.com/advisories/GHSA-mxxr-jv3v-6pgcghsaADVISORY
- nvd.nist.gov/vuln/detail/CVE-2025-62800ghsaADVISORY
- github.com/jlowin/fastmcp/commit/2a20f54617a37213ed83894a8c2f0ac38a2e83a3ghsaWEB
- github.com/jlowin/fastmcp/pull/2090ghsaWEB
- github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgcghsax_refsource_CONFIRMWEB
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