CVE-2025-53366: MCP Python SDK FastMCP Server Denial of Service Vulnerability

# CVE-2025-53366: MCP Python SDK FastMCP Server Denial of Service Vulnerability

https://gcdradar.vercel.app/incidents/cve-2025-53366-mcp-python-sdk-fastmcp-server-denial-of-servi · AIR-2025-0305 · closed 2025-07-04

- Lane: Patch now
- Severity: 2 (Disclosed vulnerability)
- Fix status: Fix status unknown
- First reported: 2025-07-04
- AI role: Attack on AI
- Products: mcp, fastmcp server
- CVE ids: CVE-2025-53366
- CVSS: 8.7

## Summary

A validation error within the FastMCP server component triggers unhandled exceptions upon processing malformed requests. This results in repeated server errors and service unavailability until restarted.

## What to do

No action recorded; read the sources below and the vendor's advisory.

## Latest updates

- 2025-07-04: CVE-2025-53366: The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). (https://nvd.nist.gov/vuln/detail/CVE-2025-53366)
- 2025-07-04: CVE-2025-53366: MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS (https://github.com/advisories/GHSA-3qhf-m339-9g5v)

## Sources

- 2025-07-04 GitHub Advisory Database: CVE-2025-53366: MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS https://github.com/advisories/GHSA-3qhf-m339-9g5v
- 2025-07-04 NVD: CVE-2025-53366: The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). https://nvd.nist.gov/vuln/detail/CVE-2025-53366

Verify against the vendor's advisory before changing production. Data CC BY-SA 4.0, https://gcdradar.vercel.app.