---
id: CVE-2025-53366
aliases:
  - GHSA-3qhf-m339-9g5v
  - PYSEC-2026-1616
title: >-
  MCP Python SDK vulnerability in the FastMCP Server causes validation error,
  leading to DoS
summary: >-
  MCP Python SDK vulnerability in the FastMCP Server causes validation error,
  leading to DoS
severity: high
vendor: mcp
product: mcp
ecosystem: pip
affected:
  - mcp < 1.9.4
patched:
  - mcp 1.9.4
published: '2025-07-04'
updated: '2026-07-07'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-3qhf-m339-9g5v'
references:
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2025-53366'
  - url: 'https://github.com/modelcontextprotocol/python-sdk/pull/822'
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
  - url: 'https://github.com/modelcontextprotocol/python-sdk'
  - url: 'https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4'
tags:
  - osv
  - pip
epss: 0.07261
epssPercentile: 0.94073
ingestedAt: '2026-07-08T18:25:44.945Z'
---

## Overview

A validation error in the MCP SDK can cause an unhandled exception when processing malformed requests, resulting in service unavailability (500 errors) until manually restarted. Impact may vary depending on the deployment conditions, and presence of infrastructure-level resilience measures.

Thank you to Rich Harang for reporting this issue.

## Affected packages

- `mcp < 1.9.4`

## Remediation

Upgrade to a patched release:

- `mcp 1.9.4`