CVE-2025-53366: Model Context Protocol Python SDK DoS in FastMCP Server

Executive brief

A vulnerability in the Model Context Protocol (MCP) Python SDK's FastMCP server can allow an attacker to crash the service. By sending specifically malformed requests, an attacker can trigger an unhandled error that makes the server unavailable to legitimate users. This results in a denial-of-service condition that typically requires a manual restart to recover.

Technical details

The MCP Python SDK (specifically the FastMCP server component) contains a vulnerability classified as an Uncaught Exception (CWE-248). A validation error occurs when the server processes malformed requests, leading to an unhandled exception that crashes the service or causes persistent 500 errors. The attack can be carried out over the network without any prior authentication or user interaction. Successful exploitation results in a complete loss of availability for the affected service until it is manually restarted. The issue is addressed in version 1.9.4 of the mcp pip package.

Affected products

• Model Context Protocol mcp < 1.9.4

Timeline

• 2025-07-04: disclosed
• 2025-07-04: advisory
• 2025-07-04: patched: Version 1.9.4 released

References

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• https://api.github.com/users/rharang/repos
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