MCP SDKsGHSA-3qhf-m339-9g5v
MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS
HighCVE-2025-53366 · Published Jul 4, 2025 · updated Jul 16, 2026
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.
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| mcp PyPI | < 1.9.4 | 1.9.4 |
Details and references
- CVSS 4.0
- CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- Severity from
- GitHub (reviewed advisory)
- Weakness
- CWE-248
- Also known as
- CVE-2025-53366, PYSEC-2026-1616
- github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v
- nvd.nist.gov/vuln/detail/CVE-2025-53366
- github.com/modelcontextprotocol/python-sdk/pull/822
- github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
- github.com/advisories/GHSA-3qhf-m339-9g5v
- github.com/modelcontextprotocol/python-sdk
- github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4
- github.com/pypa/advisory-database/tree/main/vulns/mcp/PYSEC-2026-1616.yaml
- pypi.org/project/mcp
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