GHSA-84m7-p3x7-pcfv: High severity pip/mcp vulnerability

Summary In affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state:

- sessions did not expire; - a client DELETE ended the session but left its entry registered; - a session whose first request was refused stayed registered with a running task.

Each session also kept its parsed initialize parameters, and the number of sessions was unlimited. A client that could reach the endpoint could grow server memory without bound using ordinary short requests; only a restart reclaimed it.

Am I affected? Yes, if you serve MCP over Streamable HTTP in stateful mode and clients you do not fully trust can reach the endpoint. Stateful mode is the default for:

- FastMCP (1.x) - MCPServer and Server.streamablehttpapp() (2.x) - StreamableHTTPSessionManager

Affected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1.

Not affected:

- servers on 1.12.0 or later passing statelesshttp=True (no sessions are kept) - servers using only the stdio or SSE transports

Authentication limits who can open sessions, not how many a token holder can open.

Fix Upgrade to mcp 1.30.0 (1.x) or 2.2.0 (2.x) or later. In those versions:

- a session with no request in flight for sessionidletimeout seconds (default 1800) is closed and its ID then answers 404; open streams and in-progress requests keep it alive; - at most maxsessions sessions exist at once (default 10000); a request that would open another is answered 503; - a session is discarded as soon as it ends, whether by DELETE, expiry, or a refused opening request.

Both settings are keyword arguments on FastMCP(...) (1.x), MCPServer.run() / streamablehttpapp() (2.x) and StreamableHTTPSessionManager; None disables them.

This bounds retention rather than eliminating it: a live session still holds its initialize parameters (up to roughly twice the 4 MiB body limit), and an initialize with invalid parameters still opens a session that expires like any other. On a small host, lower sessionidletimeout, maxsessions and maxrequestbodysize to fit.

If you cannot upgrade yet, pass statelesshttp=True (1.12.0 or later) where you need no per-session state, or restrict who can reach the endpoint.