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.
Cross-site Scripting
ProviderTypeBase ScoreAtk. VectorAtk. ComplexityPriv. RequiredVector
NISTNIST
6.1 MEDIUM
NETWORK
LOW
NONE
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
GitHub_MCNA
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CISA-ADPADP
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Base Score
CVSS 3.x
EPSS Score
Percentile: 17%
VendorProductVersion
jlowinfastmcp
𝑥
< 2.13.0
𝑥
= Vulnerable software versions
Common Weakness Enumeration
References
https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc
https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc