High severity · published Jul 4, 2025
CVE-2025-53366
MCP SDK Vulnerable to FastMCP Server Validation Error, Leading to Denial of Service
8.7
CVSS
9.8%
EPSS
No
On KEV
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.
Exploitation
Is it being used?
Exploit prediction (EPSS)9.8% · higher than 96% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
PY
python-sdk · modelcontextprotocol
< 1.9.4 · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-53366
CVE.org
https://www.cve.org/CVERecord?id=CVE-2025-53366
github.com
https://github.com/modelcontextprotocol/python-sdk/commit/29c69e6a47d0104d0afcea6ac35e7ab02fde809a
github.com
https://github.com/modelcontextprotocol/python-sdk/pull/822
github.com
https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-3qhf-m339-9g5v
More from python-sdk
8.7
CVE-2025-53365
MCP Python SDK has Unhandled Exception in Streamable HTTP Transport ,Leading to Denial of Service
8.1
CVE-2026-59950
MCP Python SDK: WebSocket server transport does not support Host/Origin validation
8.1
CVE-2025-66416
DNS Rebinding Protection Disabled by Default in Model Context Protocol Python SDK for Servers Running on Localhost
7.6
CVE-2026-52870
MCP Python SDK: Experimental task handlers allow any client to access and cancel other clients' tasks
7.5
CVE-2026-109527
Model Context Protocol Python SDK HTTP and OAuth endpoints allow memory exhaustion through unbounded request bodies