CVE-2025-53366 MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service
Attackers can cause service unavailability by sending malformed requests that trigger unhandled exceptions in the MCP Python SDK.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-53366?
CVE-2025-53366 is a high-severity denial of service vulnerability in modelcontextprotocol python-sdk, published on 4 Jul 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-248
Which versions are affected by CVE-2025-53366?
3 open source packages ship an affected version, including mcp.
| Package | Affected | Fixed in |
|---|---|---|
mcp PyPI | All versions | 1.9.4 |
mcp PyPI | All versions | 1.9.4 |
mcp PyPI | All versions | 1.9.4 |
How do I fix CVE-2025-53366?
Upgrade mcp to 1.9.4 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-53366 being exploited?
There is no known exploitation of CVE-2025-53366 in the wild and no public exploit code.
Sources
Also known as GHSA-3qhf-m339-9g5v, PYSEC-2026-1616, ROOT-APP-PYPI-CVE-2025-53366.