MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion

### 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.streamable_http_app()` (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 `stateless_http=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 `session_idle_timeout` seconds (default 1800) is closed and its ID then answers `404`; open streams and in-progress requests keep it alive; - at most `max_sessions` 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()` / `streamable_http_app()` (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 `session_idle_timeout`, `max_sessions` and `max_request_body_size` to fit. If you cannot upgrade yet, pass `stateless_http=True` (1.12.0 or later) where you need no per-session state, or restrict who can reach the endpoint.

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-400, CWE-770, CWE-772

Also known as

CVE-2026-59951

• github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv
• nvd.nist.gov/vuln/detail/CVE-2026-59951
• github.com/modelcontextprotocol/python-sdk/pull/3395
• github.com/modelcontextprotocol/python-sdk/pull/3426
• github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932fc1d994e7
• github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e6575931b
• github.com/modelcontextprotocol/python-sdk
• github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0
• github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0