{"schema_version":"1.9.0","id":"GHSA-84m7-p3x7-pcfv","published":"2026-10-15T20:13:13Z","modified":"2026-10-15T20:30:08.565001293Z","aliases":["CVE-2026-59951"],"summary":"MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion","details":"### Summary\nIn affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state:\n\n- sessions did not expire;\n- a client `DELETE` ended the session but left its entry registered;\n- a session whose first request was refused stayed registered with a running task.\n\nEach 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.\n\n### Am I affected?\nYes, 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:\n\n- `FastMCP` (1.x)\n- `MCPServer` and `Server.streamable_http_app()` (2.x)\n- `StreamableHTTPSessionManager`\n\nAffected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1.\n\nNot affected:\n\n- servers on 1.12.0 or later passing `stateless_http=True` (no sessions are kept)\n- servers using only the stdio or SSE transports\n\nAuthentication limits who can open sessions, not how many a token holder can open.\n\n### Fix\nUpgrade to `mcp` 1.30.0 (1.x) or 2.2.0 (2.x) or later. In those versions:\n\n- 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;\n- at most `max_sessions` sessions exist at once (default 10000); a request that would open another is answered `503`;\n- a session is discarded as soon as it ends, whether by `DELETE`, expiry, or a refused opening request.\n\nBoth settings are keyword arguments on `FastMCP(...)` (1.x), `MCPServer.run()` / `streamable_http_app()` (2.x) and `StreamableHTTPSessionManager`; `None` disables them.\n\nThis 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.\n\nIf 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.","affected":[{"package":{"name":"mcp","ecosystem":"PyPI","purl":"pkg:pypi/mcp"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"1.8.0"},{"fixed":"1.30.0"}]}],"versions":["1.10.0","1.10.1","1.11.0","1.12.0","1.12.1","1.12.2","1.12.3","1.12.4","1.13.0","1.13.1","1.14.0","1.14.1","1.15.0","1.16.0","1.17.0","1.18.0","1.19.0","1.20.0","1.21.0","1.21.1","1.21.2","1.22.0","1.23.0","1.23.1","1.23.2","1.23.3","1.24.0","1.25.0","1.26.0","1.27.0","1.27.1","1.27.2","1.28.0","1.28.1","1.29.0","1.29.1","1.8.0","1.8.1","1.9.0","1.9.1","1.9.2","1.9.3","1.9.4"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-84m7-p3x7-pcfv/GHSA-84m7-p3x7-pcfv.json"}},{"package":{"name":"mcp","ecosystem":"PyPI","purl":"pkg:pypi/mcp"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.0.0"},{"fixed":"2.2.0"}]}],"versions":["2.0.0","2.0.1","2.1.0","2.1.1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-84m7-p3x7-pcfv/GHSA-84m7-p3x7-pcfv.json"}}],"references":[{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-59951"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/pull/3395"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/pull/3426"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932fc1d994e7"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e6575931b"},{"type":"PACKAGE","url":"https://github.com/modelcontextprotocol/python-sdk"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0"}],"database_specific":{"cwe_ids":["CWE-400","CWE-770","CWE-772"],"github_reviewed":true,"github_reviewed_at":"2026-10-15T20:13:13Z","nvd_published_at":"2026-10-14T17:18:29Z","severity":"MODERATE"},"severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"}]}