CVE-2026-63128: RMCP: Unauthenticated permanent session-table leak in rmcp Streamable HTTP server transport leads to remote denial-of-service

An unauthenticated remote attacker can leak one entry per HTTP request out of the in-memory session table of LocalSessionManager by sending a well-formed JSON-RPC POST that is not an InitializeRequest. The Streamable HTTP server’s handle_post allocates the session before it validates the body, then early-returns on the validation failure without calling close_session. The LocalSessionHandle (and the tokio mpsc channel internals it holds) is never released for the remainder of the process’s lifetime — turning a ~250-byte request into a permanent ~400–550-byte server-side allocation that scales linearly with request volume and eventually exhausts memory. In the verified reproduction below, a single Python client sustains over 2 000 leak requests per second; that translates to roughly 170 million leaked entries per day, equivalent to ≈75 GB of resident memory just from the session table.