CVE-2026-59951

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

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-59951 has a CVSS score of 7.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.30.0, 2.2.0); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to mcp 1.30.0 (1.x) or 2.2.0 (2.x) or later. In those versions:

• a session with no request in flight for session_idle_timeout seconds (default 1800) is closed and its ID then answers 404; open streams and in-progress requests keep it alive;
• at most max_sessions sessions exist at once (default 10000); a request that would open another is answered 503;
• a session is discarded as soon as it ends, whether by DELETE, expiry, or a refused opening request.

Both settings are keyword arguments on FastMCP(...) (1.x), MCPServer.run() / streamable_http_app() (2.x) and StreamableHTTPSessionManager; None disables them.

This bounds retention rather than eliminating it: a live session still holds its initialize parameters (up to roughly twice the 4 MiB body limit), and an initialize with invalid parameters still opens a session that expires like any other. On a small host, lower session_idle_timeout, max_sessions and max_request_body_size to fit.

If you cannot upgrade yet, pass stateless_http=True (1.12.0 or later) where you need no per-session state, or restrict who can reach the endpoint.

1. What is CVE-2026-59951? CVE-2026-59951 is a medium-severity uncontrolled resource consumption vulnerability in mcp (pip), affecting versions >= 1.8.0, < 1.30.0. It is fixed in 1.30.0, 2.2.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-59951? CVE-2026-59951 has a CVSS score of 7.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of mcp are affected by CVE-2026-59951? mcp (pip) versions >= 1.8.0, < 1.30.0 is affected.
4. Is there a fix for CVE-2026-59951? Yes. CVE-2026-59951 is fixed in 1.30.0, 2.2.0. Upgrade to this version or later.
5. Is CVE-2026-59951 exploitable, and should I be worried? Whether CVE-2026-59951 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-59951 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-59951?
   • Upgrade mcp to 1.30.0 or later
   • Upgrade mcp to 2.2.0 or later

Other vulnerabilities in mcp