CVE-2026-59951: MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion

What it is

In affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state:

In affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state:

- sessions did not expire; - a client DELETE ended the session but left its entry registered; - a session whose first request was refused stayed registered with a running task.

Each 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.

How to check

1. Affected: mcp (pip) >= 1.8.0, < 1.30.0.
2. Affected: mcp (pip) >= 2.0.0, < 2.2.0.
3. Compare your installed version with the range above (command below).

How to fix

1. Upgrade mcp to 1.30.0 or later.
2. Upgrade mcp to 2.2.0 or later.

pip show mcp | grep Version

Sources

• GitHub Security Advisory GHSA-84m7-p3x7-pcfv ↗
• NVD: CVE-2026-59951 ↗