CVE-2025-53366: MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS
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.
Published Jul 4, 2025 · Updated Jul 16, 2026
Affected packages
mcp
- Ecosystem
- PyPI
- Affected
- < 1.9.4
- Fixed in
- 1.9.4
- Check
pip show mcp
Advisory details
From the GitHub Advisory Database (CC BY 4.0).
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.
Credited to rharang.
References
- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v
- https://github.com/modelcontextprotocol/python-sdk/pull/822
- https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4
- https://nvd.nist.gov/vuln/detail/CVE-2025-53366
- https://github.com/advisories/GHSA-3qhf-m339-9g5v
- https://github.com/modelcontextprotocol/python-sdk
- https://github.com/pypa/advisory-database/tree/main/vulns/mcp/PYSEC-2026-1616.yaml
- https://pypi.org/project/mcp
How this page is made: the data panel comes from the GitHub Advisory Database, CISA’s KEV catalog, the NVD and FIRST EPSS, and updates twice a day. No analysis has been written for this entry yet. Methodology
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