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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | mcp | < 1.9.4 | 1.9.4 |
References
- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v
- https://nvd.nist.gov/vuln/detail/CVE-2025-53366
- https://github.com/modelcontextprotocol/python-sdk/pull/822
- https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
- https://github.com/modelcontextprotocol/python-sdk
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.9.4
- https://pypi.org/project/mcp
- https://github.com/advisories/GHSA-3qhf-m339-9g5v
- https://github.com/pypa/advisory-database/tree/main/vulns/mcp/PYSEC-2026-1616.yaml
- CVSS vector
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- Weakness (CWE)
CWE-248- Platform
- Python (PyPI)
- Package
mcp- Disclosed
- 04.07.2025
- Updated
- 11.10.2026 14:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1616 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-3qhf-m339-9g5v - CC BY 4.0
Vulnerability history: mcp
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 6
- First → latest
- 07.2025 07.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 6
Disclosures per year
- 2025: 3
- 2026: 3
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MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS this entry
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