What’s Vulnerable › Vulnerabilities

CVE-2025-53366: MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service

• Severity: high (CVSS 8.7 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 9.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-07-04 · updated 2026-06-17
• Weakness: CWE-248

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.

Affected products

• modelcontextprotocol python-sdk: before 1.9.4

References

• https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
• https://github.com/modelcontextprotocol/python-sdk/pull/822
• https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v

Get alerted when CVE-2025-53366 or the next vulnerability affects your stack