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TI-2025-38983 Aliases CVE-2025-62800GHSA-mxxr-jv3v-6pgc

FastMCP vulnerable to reflected XSS in client's callback page

Priority MEDIUM 57CWE-79Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
Passive
Attack complexity
Low
Attack requirements
None
Impact type
Cross-site scripting
Confidentiality · integrity · availability
None · None · None
Fix status
Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub. Impact type from CWE-79.

Summary

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.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00276 20th percentileScored 2026-11-07
Public exploit Yes 1 exploit referenceTagged by NVD
Fix Fixed fastmcp (jlowin): 2.13.0fastmcp (PyPI): 2.13.0

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
jlowinfastmcpbefore 2.13.02.13.0Patch availableCNA
—fastmcp (PyPI)before 2.13.02.13.0Patch availableGHSA-mxxr-jv3v-6pgc
  • fastmcp Vendorjlowin Affected before 2.13.0 Fixed in 2.13.0 Patch available CNA
  • fastmcp (PyPI) Vendor— Affected before 2.13.0 Fixed in 2.13.0 Patch available GHSA-mxxr-jv3v-6pgc

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*before 2.13.0Vulnerablejlowin
  • cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:* Versions before 2.13.0 Vulnerable ·jlowin

Remediation

  • Fix · CNA fastmcp (jlowin) Fixed in 2.13.0
  • Fix · GHSA-mxxr-jv3v-6pgc fastmcp (PyPI) Fixed in 2.13.0 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2025-62800
  2. 2026-10-03 Alias added: GHSA-mxxr-jv3v-6pgc

References

Exploit 1

Advisory 1

Other 3

  • TI-2026-25332 FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability CVSS 4.0CRITICAL 10.0
  • TI-2026-21575 FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities CVSS 4.0HIGH 8.2
  • TI-2025-39774 FastMCP has a Command Injection vulnerability - Gemini CLI CVSS 3.1HIGH 7.8
  • TI-2025-42650 FastMCP OAuth Proxy token reuse across MCP servers CVSS 4.0HIGH 7.4
  • TI-2025-38984 FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name CVSS 4.0MEDIUM 5.4

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences