{"cve":"CVE-2026-59951","nvd":{"published":"2026-10-14T17:18:29.690","modified":"2026-10-16T16:00:33.533","status":"Awaiting Analysis","description":"The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). From 1.8.0 until 1.30.0 and 2.2.0, stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager, retain session entries without an idle expiration or aggregate session-count limit. DELETE, refused opening requests, and completed sessions do not reliably discard their entries, and each entry can retain parsed initialize parameters. A client that can reach the endpoint can therefore create ordinary short requests that grow server memory without bound until restart. Stateless HTTP, stdio, and SSE transports are not affected. This issue is fixed in versions 1.30.0 and 2.2.0.","cvss_v4":{"vector":"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","score":6.9},"cvss_v3":null,"cwes":["CWE-400","CWE-772"],"references":["https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932fc1d994e7","https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e6575931b","https://github.com/modelcontextprotocol/python-sdk/pull/3395","https://github.com/modelcontextprotocol/python-sdk/pull/3426","https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0"],"products":[],"source":"nvd","synced_at":"2026-10-19T22:59:28.494260111+00:00"},"epss":{"cve":"CVE-2026-59951","score":0.00458,"percentile":0.39447,"date":"2026-10-25"},"kev":null,"osv":[{"id":"GHSA-84m7-p3x7-pcfv","url":"https://osv.dev/vulnerability/GHSA-84m7-p3x7-pcfv","summary":"MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion","modified":"2026-10-15T20:30:08.565001293Z","packages":[{"ecosystem":"PyPI","name":"mcp","purl":"pkg:pypi/mcp","ranges":[">=1.8.0 <1.30.0"],"fixed":["1.30.0"]},{"ecosystem":"PyPI","name":"mcp","purl":"pkg:pypi/mcp","ranges":[">=2.0.0 <2.2.0"],"fixed":["2.2.0"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}