MCP Python SDK vulnerabilities

The MCP Python SDK (mcp on PyPI) from 1.8.0 until 1.30.0 and 2.2.0 does not reliably discard session entries in stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager. A client that can reach the endpoint can send ordinary short requests that grow server memory without bound until restart. Stateless HTTP, stdio, and SSE transports are not affected.

A vulnerability in the MCP Python SDK (mcp) prior to version 1.28.1 affects the deprecated WebSocket transport, which did not validate Host or Origin headers. This allowed arbitrary third parties to establish WebSocket connections to applications using that transport.

A vulnerability in the MCP Python SDK (mcp) versions 1.23.0 through 1.27.2 allows any client to access other clients' tasks. Default handlers for task operations do not verify the session, enabling enumeration, reading results, consuming messages, or canceling tasks of other clients.

A vulnerability in MCP Python SDK (mcp) before version 1.27.2 allows an attacker to inject JSON-RPC messages into an existing session. The issue affects SSE and Streamable HTTP transports, which do not verify the authenticated client's identity when using the session_id parameter or Mcp-Session-Id header.