CVE-2025-53366 — MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service
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 …
Published: 2025-07-04 · CVSS: 8.7 · CVSS 4.0
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.
Source: CVE Program · Revision 8b16575c39f1e1810c5e863010da073d2d3285e7