Snyk agent scan: MCP Python SDK: WebSocket server transport does not support Host/Origin validation (CVE-2026-59950)

In affected versions (>=0.4.3 <0.5.17) of the MCP Python SDK, the deprecated WebSocket server transport (mcp.server.websocket.websocket_server) accepts WebSocket handshakes without validating the Host or Origin headers. Unlike other transports that use TransportSecuritySettings for validation, this transport immediately accepts connections, allowing cross-origin web pages to establish sessions and issue JSON-RPC requests without authentication or origin checks. This can expose server tools and resources if the server is bound to localhost or LAN and lacks additional access controls. The vulnerability is addressed in version 1.28.1 by adding support for TransportSecuritySettings validation in the WebSocket transport, rejecting invalid requests with HTTP 403. However, validation is disabled by default and must be explicitly enabled. The best practice is to migrate to Streamable HTTP transport, which enforces these protections automatically. The deprecated WebSocket transport is removed in version 2.

An attacker can use a malicious web page to connect to an MCP server using the deprecated WebSocket transport without authentication or origin validation, potentially enumerating and invoking server tools and accessing resources exposed by the server. The impact depends on what the server exposes. Browsers may prompt users before allowing connections to local network addresses, but this is not a substitute for server-side validation. This vulnerability can lead to unauthorized access and control over the MCP server's exposed functionality.

Upgrade to MCP Python SDK version 1.28.1 or later, which adds optional Host and Origin header validation to the deprecated WebSocket transport. To enable protection, pass a TransportSecuritySettings object with enable_dns_rebinding_protection set to true and configure allowed_hosts and allowed_origins appropriately. Simply upgrading without enabling validation does not mitigate the issue. The recommended mitigation is to migrate off the deprecated WebSocket transport to the Streamable HTTP transport, where FastMCP automatically enforces these security checks. Note that the WebSocket transport is removed entirely in version 2, so migration is advised.