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

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

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.

References

modelcontextprotocol/python-sdk security advisory · WEB
nvd.nist.gov/vuln/detail/CVE-2025-53366 · ADVISORY
modelcontextprotocol/python-sdk#822 · WEB
modelcontextprotocol/python-sdk commit 29c69e6 · WEB
GitHub Advisory GHSA-3qhf-m339-9g5v · ADVISORY
modelcontextprotocol/python-sdk · PACKAGE
modelcontextprotocol/python-sdk release v1.9.4 · WEB
pypa/advisory-database · WEB
pypi.org/project/mcp · WEB

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Package

Affected versions

Patched versions

mcp

PyPI

< 1.9.4

1.9.4

Description

Thank you to Rich Harang for reporting this issue.

NVD description