CVE-2026-59950: MCP Python SDK: WebSocket server transport does not support Host/Origin validation

What it is

In affected versions, the deprecated WebSocket server transport (mcp.server.websocket.websocket_server) accepted the WebSocket handshake without applying any Host or Origin header validation. The TransportSecuritySettings mechanism that the SSE and Streamable HTTP transports use for this purpose was not wired into the WebSocket transport, so there was no SDK-level way to restrict which origins could connect.

Am I affected? Only if a developer's application server exposes mcp.server.websocket.websocket_server. This transport has never been part of the MCP specification, is marked deprecated, and is not reachable through FastMCP — a developer must have wired it into an ASGI application themselves. Servers using stdio, SSE, or Streamable HTTP are not affected by this advisory.

websocket_server() constructed a Starlette WebSocket and called accept(subprotocol="mcp") immediately, with no inspection of the connection's headers. By contrast, SseServerTransport and StreamableHTTPServerTransport accept an optional security_settings: TransportSecuritySettings and run TransportSecurityMiddleware.validate_request() against the incoming Host and Origin headers before establishing a session.

How to check

1. Affected: mcp (pip) < 1.28.1.
2. Compare your installed version with the range above (command below).

How to fix

1. Upgrade mcp to 1.28.1 or later.

pip show mcp | grep Version

Sources

• GitHub Security Advisory GHSA-vj7q-gjh5-988w ↗
• NVD: CVE-2026-59950 ↗