CVE-2026-59951

Name of the Vulnerable Software and Affected Versions mcp versions 1.8.0 through 1.29.9 mcp versions 2.0.0 through 2.1.9

Description 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. Because DELETE requests, refused opening requests, and completed sessions do not reliably discard entries, and each entry can retain parsed initialize parameters, a client can send short requests to the endpoint to increase server memory consumption without bound until a restart occurs. Stateless HTTP, stdio, and SSE transports are not affected.

Recommendations Update to version 1.30.0. Update to version 2.2.0.