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MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS
Canonical as of Jul 4, 2025
MCP Python SDK vulnerability in the FastMCP Server causes validation error, leading to DoS
High severity. Affects mcp < 1.9.4. Upgrade to 1.9.4 or later.
CVE-2025-53366 / GHSA-3qhf-m339-9g5v · severity: high · PyPI
Affected
mcp< 1.9.4 → fixed in 1.9.4
Details
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.
Source: GHSA-3qhf-m339-9g5v — GitHub Advisory Database (CC-BY-4.0).