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

Source: https://junglewise.ai/threats/cve-2025-53366-model-context-protocol-python-sdk-dos-in-fastmcp-server  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-07-04. Updated: 2026-07-16  
License: CC BY 4.0 with a link back

CVE-2025-53366 · Severity: high · CVSS 4 · Published 2025-07-04

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.

Technologies: [mcp (PyPI)](https://junglewise.ai/threats/technologies/pypi-mcp). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2025-53366 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2025-53366 being exploited in the wild.

**How severe is CVE-2025-53366?** High severity, with a CVSS score of 4, published 4 July 2025.

**Which products and versions are affected?** mcp (PyPI) ECOSYSTEM:introduced=0,fixed=1.9.4.

**Is there a fix for CVE-2025-53366?** Yes. mcp (PyPI): upgrade to 1.9.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2025-53366 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2025-53366?** Check whether you run mcp (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- mcp (PyPI): upgrade to 1.9.4 or later

  ```sh
  pip install "mcp>=1.9.4"
  ```


## 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

## Identifiers

- CVE-2025-53366
- GHSA-3qhf-m339-9g5v
- PYSEC-2026-1616

## Timeline

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

## References

- [https://api.github.com/users/rharang](https://api.github.com/users/rharang)
- [https://github.com/rharang](https://github.com/rharang)
- [https://api.github.com/users/rharang/gists%7B/gist_id%7D](https://api.github.com/users/rharang/gists%7B/gist_id%7D)
- [https://api.github.com/users/rharang/repos](https://api.github.com/users/rharang/repos)
- [https://avatars.githubusercontent.com/u/8082617?v=4](https://avatars.githubusercontent.com/u/8082617?v=4)
- [https://api.github.com/users/rharang/events%7B/privacy%7D](https://api.github.com/users/rharang/events%7B/privacy%7D)

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