Is there a public exploit for CVE-2026-59951?

No public exploit for CVE-2026-59951 was found in Exploit-DB, Metasploit, Nuclei templates or public proof-of-concept repositories. That is an absence of evidence rather than proof that none exists.

How severe is CVE-2026-59951?

CVE-2026-59951 scores CVSS 6.9 (v4.0), rated medium. EPSS puts the chance of exploitation in the next 30 days at 0.46%.

What does CVE-2026-59951 affect?

CVE-2026-59951 affects modelcontextprotocol mcp. It is part of the AI and agent stack (Model Context Protocol).

6.9 Medium

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP).

CVE-2026-59951

Summary 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. Am I affected? Yes, if you serve MCP over Streamable HTTP in stateful mode and clients you do not fully trust can reach the endpoint. Stateful mode is the default for: FastMCP (1.x) MCPServer and Server.streamablehttpapp() (2.x) StreamableHTTPSessionManager Affected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1. Not affected: servers on 1.12.0 or later passing statelesshttp=True (no sessions are kept) servers using only the stdio or SSE transports Authentication limits who can open sessions, not how many a token holder can open. Fix 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…

Part of the AI stack

This affects Model Context Protocol, in MCP & tool servers.

Assessment

CVSS: 6.9 (v4.0)
Severity: Medium
EPSS: 0.46% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 14 October 2026
Last updated: 16 October 2026
Affected: modelcontextprotocol · python-sdk · mcp
Weakness: CWE-400, CWE-770, CWE-772
Assigner: security-advisories@github.com

CVSS vectorCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

At the source

References

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