{"schema":"threatline.record/2","record":{"id":"CVE-2026-59951","aliases":["GHSA-84m7-p3x7-pcfv"],"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.","categories":["Libraries & packages"],"product":"mcp","vendor":"modelcontextprotocol","ecosystem":"pip","affected_ranges":[{"source":"GitHub Advisory Database","source_url":"https://github.com/advisories/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:13:14Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"pip","name":"mcp"},"range":">= 1.8.0, < 1.30.0"},{"source":"GitHub Advisory Database","source_url":"https://github.com/advisories/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:13:14Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"pip","name":"mcp"},"range":">= 2.0.0, < 2.2.0"},{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-59951","source_modified_at":"2026-10-15T15:07:43.737Z","fetched_at":"2026-10-23T18:45:31.882637Z","product":"python-sdk","vendor":"modelcontextprotocol","versions":[{"version":">= 1.8.0, < 1.30.0","status":"affected"},{"version":">= 2.0.0, < 2.2.0","status":"affected"}],"defaultStatus":null},{"source":"OSV","source_url":"https://osv.dev/vulnerability/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:30:08.565001293Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"PyPI","name":"mcp","purl":"pkg:pypi/mcp"},"ranges":[{"events":[{"introduced":"1.8.0"},{"fixed":"1.30.0"}],"type":"ECOSYSTEM"}],"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"]},{"source":"OSV","source_url":"https://osv.dev/vulnerability/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:30:08.565001293Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"PyPI","name":"mcp","purl":"pkg:pypi/mcp"},"ranges":[{"events":[{"introduced":"2.0.0"},{"fixed":"2.2.0"}],"type":"ECOSYSTEM"}],"versions":["2.0.0","2.0.1","2.1.0","2.1.1"]}],"fixed_versions":[{"source":"GitHub Advisory Database","source_url":"https://github.com/advisories/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:13:14Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"pip","name":"mcp"},"version":"1.30.0"},{"source":"GitHub Advisory Database","source_url":"https://github.com/advisories/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:13:14Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"pip","name":"mcp"},"version":"2.2.0"},{"source":"OSV","source_url":"https://osv.dev/vulnerability/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:30:08.565001293Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"PyPI","name":"mcp","purl":"pkg:pypi/mcp"},"version":"1.30.0"},{"source":"OSV","source_url":"https://osv.dev/vulnerability/GHSA-84m7-p3x7-pcfv","source_modified_at":"2026-10-15T20:30:08.565001293Z","fetched_at":"2026-10-23T18:42:43.628194Z","package":{"ecosystem":"PyPI","name":"mcp","purl":"pkg:pypi/mcp"},"version":"2.2.0"}],"cvss":6.9,"severity":"Medium","kev":{"listed":false},"epss":{"probability":0.00458,"percentile":0.38724,"score_date":"2026-10-23"},"published_at":"2026-10-14T16:22:31.653Z","modified_at":"2026-10-15T20:30:08.565001293Z","provenance":[{"source":"GitHub Advisory Database","url":"https://github.com/advisories/GHSA-84m7-p3x7-pcfv","fetched_at":"2026-10-23T18:42:43.628194Z","source_modified_at":"2026-10-15T20:13:14Z"},{"source":"CVE List","url":"https://www.cve.org/CVERecord?id=CVE-2026-59951","fetched_at":"2026-10-23T18:45:31.882637Z","source_modified_at":"2026-10-15T15:07:43.737Z"},{"source":"OSV","url":"https://osv.dev/vulnerability/GHSA-84m7-p3x7-pcfv","fetched_at":"2026-10-23T18:42:43.628194Z","source_modified_at":"2026-10-15T20:30:08.565001293Z"},{"source":"EPSS","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-59951","fetched_at":"2026-10-23T18:42:43.628194Z","source_modified_at":"2026-10-23"}],"sources":["GitHub Advisory Database","CVE List","OSV","EPSS"],"references":[{"source":"CVE List","url":"https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv","tags":["x_refsource_CONFIRM"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-59951","source":"GitHub Advisory Database"},{"source":"CVE List","url":"https://github.com/modelcontextprotocol/python-sdk/pull/3395","tags":["x_refsource_MISC"]},{"source":"CVE List","url":"https://github.com/modelcontextprotocol/python-sdk/pull/3426","tags":["x_refsource_MISC"]},{"source":"CVE List","url":"https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932fc1d994e7","tags":["x_refsource_MISC"]},{"source":"CVE List","url":"https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e6575931b","tags":["x_refsource_MISC"]},{"source":"CVE List","url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0","tags":["x_refsource_MISC"]},{"source":"CVE List","url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0","tags":["x_refsource_MISC"]},{"url":"https://github.com/advisories/GHSA-84m7-p3x7-pcfv","source":"GitHub Advisory Database"}],"locations":[],"motoko_assessment":{"status":"requires_architecture_review","automatic_remediation":false},"cve_state":"PUBLISHED","ssvc":{"exploitation":"none","automatable":"yes","technical_impact":"partial","assessed_at":"2026-10-15T13:55:54.339140Z"},"classification":{"method":"product-first software-layer rules","rules_version":"2.1","matched_on":"ecosystem","confidence":"heuristic","reviewed":false},"change_timestamp":"2026-10-15T20:30:08.565001293Z","content_hash":"f175c396db66c67abf8e79852f4587039d32d61a44e6b18edb6a0edade738a7e"}}