CVE-2025-53366
MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS
EPSS 0.19%
Description
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. Thank you to Rich Harang for reporting this issue.
How to fix CVE-2025-53366
To remediate CVE-2025-53366, upgrade the affected package to a fixed version below.
- PyPI/mcp—upgrade to 1.9.4 or later
Is CVE-2025-53366 being exploited?
Low — EPSS is 0.2%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0, < 1.9.4
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | 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 |