CVE-2025-53366

MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service

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.

Category

8.7
CVSS
Severity: High
CVSS 4.0 •
EPSS 0.04%
Affected: modelcontextprotocol python-sdk
Published at:
Updated at:

References

Link Tags
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

Frequently Asked Questions

What is the severity of CVE-2025-53366?
CVE-2025-53366 has been scored as a high severity vulnerability.
How to fix CVE-2025-53366?
To fix CVE-2025-53366, make sure you are using an up-to-date version of the affected component(s) by checking the vendor release notes. As for now, there are no other specific guidelines available.
Is CVE-2025-53366 being actively exploited in the wild?
As for now, there are no information to confirm that CVE-2025-53366 is being actively exploited. According to its EPSS score, there is a ~0% probability that this vulnerability will be exploited by malicious actors in the next 30 days.
What software or system is affected by CVE-2025-53366?
CVE-2025-53366 affects modelcontextprotocol python-sdk.
This platform uses data from the NIST NVD, MITRE CVE, MITRE CWE, First.org and CISA KEV but is not endorsed or certified by these entities. CVE is a registred trademark of the MITRE Corporation and the authoritative source of CVE content is MITRE's CVE web site. CWE is a registred trademark of the MITRE Corporation and the authoritative source of CWE content is MITRE's CWE web site.
© 2025 Under My Watch. All Rights Reserved.