MCP Python SDK advisory reports the deprecated WebSocket server transport skipped Host and Origin validation

The MCP Python SDK maintainers published advisory GHSA-vj7q-gjh5-988w (CVE-2026-59950) on July 7, rated High with a CVSS 7.6 score in the advisory record. The deprecated websocket_server() transport did not implement the TransportSecuritySettings mechanism that the SDK's other HTTP-based transports use for Host and Origin validation, so a page served from any origin could complete a WebSocket upgrade against an exposed server and, per the advisory, enumerate and invoke the server's tools and read its resources.

The blast radius is narrow by construction: only servers explicitly calling the deprecated websocket_server() function are affected, not standard FastMCP usage. A precision point for readers tracking transports: this WebSocket transport is an SDK-level extra, not one of the specification's transports (streamable-http, stdio, and the legacy SSE transport).

The fix predates the advisory by eleven days: v1.28.1, published June 26, carries PR #2992 adding TransportSecuritySettings support to the WebSocket server transport. The advisory credits five reporters. The affected package is mcp on PyPI below 1.28.1.