CVE-2025-62800— FastMCP vulnerable to reflected XSS in client's callback page
Quick assessment
Affected
jlowin fastmcp
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.
FastMCP是Jeremiah Lowin个人开发者的一个MCP服务器构建软件。 FastMCP 2.13.0之前版本存在跨站脚本漏洞，该漏洞源于OAuth客户端回调页面未转义用户控制值，可能导致反射型跨站脚本攻击。
Possible ATT&CK Techniques 1 AI
T1189 · Drive-by Compromise
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I. Basic Information for CVE-2025-62800
Vulnerability Information
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Vulnerability Title
FastMCP vulnerable to reflected XSS in client's callback page
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
在Web页面生成时对输入的转义处理不恰当(跨站脚本)
Source: CVE Program / CVE List V5
Vulnerability Title
FastMCP 跨站脚本漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
FastMCP是Jeremiah Lowin个人开发者的一个MCP服务器构建软件。 FastMCP 2.13.0之前版本存在跨站脚本漏洞，该漏洞源于OAuth客户端回调页面未转义用户控制值，可能导致反射型跨站脚本攻击。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)
Affected Products
II. Public POCs for CVE-2025-62800
| # | POC Description | Source Link | Shenlong Link |
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Vendor Advisories for CVE-2025-62800 (1)
-
Reflected XSS in client's callback page · Advisory · jlowin/fastmcp · GitHub x_refsource_CONFIRM
IV. Related Vulnerabilities
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CVE-2026-27124
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Same vendor: jlowin
Same weakness: CWE-79
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V. Comments for CVE-2025-62800
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