CVE-2026-59951
Severity not reportedThe MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP).
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.
Affected products
Not reported by the source (unknown — not "none").
Weakness classification (CWE)
Source: NVD / CISA. “NVD-CWE-Other” and “NVD-CWE-noinfo” mean NVD could not assign a specific weakness.
Mitigation & advisories
No vendor advisory or patch reference tagged by NVD. Check the references below.
References
- https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932f... security-advisories@github.com
- https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e... security-advisories@github.com
- https://github.com/modelcontextprotocol/python-sdk/pull/3395 security-advisories@github.com
- https://github.com/modelcontextprotocol/python-sdk/pull/3426 security-advisories@github.com
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0 security-advisories@github.com
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0 security-advisories@github.com
- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv security-advisories@github.com