MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS

HOCH 8.7CVE-2025-53366 · GHSA-3qhf-m339-9g5v · PYSEC-2026-1616 · Published 04/07/2025 · updated 17/06/2026

Kurz gesagt

CVE-2025-53366 ist eine Schwachstelle (HOCH (CVSS 8.7)) in mcp.
Wahrscheinlichkeit einer Ausnutzung in den nächsten 30 Tagen (EPSS): 9.8%, höher als bei 95.5% aller Schwachstellen.
Abhilfe: auf 1.9.4 oder neuer aktualisieren.

Package	Affected versions	Patched versions
mcp PyPI	`< 1.9.4`	`1.9.4`

Description

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.

NVD description

The MCP Python SDK, called `mcp` on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.9.4, 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. Version 1.9.4 contains a patch for the issue.