Vulnerability · Published Jul 4, 2025 · Assigned by GitHub_M
CVE-2025-53366MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service
High 8.7
In plain English
A high-severity security flaw in python-sdk from modelcontextprotocol. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 9.8% (higher than 96% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| PyPI | mcp | 1.9.4 | GHSA-3qhf-m339-9g5v |
| PyPI | mcp | 1.9.4 | PYSEC-2026-1616 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| modelcontextprotocol | python-sdk | < 1.9.4 |
Weakness: CWE-248 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
References
- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v x_refsource_CONFIRM
- https://github.com/modelcontextprotocol/python-sdk/pull/822 x_refsource_MISC
- https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a x_refsource_MISC