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OpenAdvisories
High · 8.7 CVE-2025-53366 EUVD-2025-20093GHSA-3qhf-m339-9g5vPYSEC-2026-1616

MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service

Published Jul 4, 2025 · Updated Jul 8, 2025 · Assigned by GitHub_M

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.

Affected products

VendorProductAffectedFixed
modelcontextprotocol python-sdk < 1.9.4 —

Affected packages

EcosystemPackageAffectedFixedRecord
PyPI mcp < 1.9.4 1.9.4 GHSA-3qhf-m339-9g5v

References