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[ CVE-2026-59951 · Published 14 Oct 2026 ]

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

CVSS 6.9 EPSS <1%Fix available

[ Description ]Official 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.

[ Fixes ]Fixes and patches

  • mcp 1.30.0Fixed version · GitHub Advisories · pip package mcp: upgrade to 1.30.0 or later (affected >= 1.8.0, < 1.30.0)
  • mcp 2.2.0Fixed version · GitHub Advisories · pip package mcp: upgrade to 2.2.0 or later (affected >= 2.0.0, < 2.2.0)

[ Packages ]Affected open-source packages

[ References ]References