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

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

En bref

CVE-2025-53366 est une vulnérabilité ÉLEVÉE (CVSS 8.7) touchant mcp.
Probabilité d'exploitation dans les 30 prochains jours (EPSS) : 9.8%, plus élevée que 95.5% des vulnérabilités.
Correctif : mettre à jour vers 1.9.4 ou plus récent.

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.