BREW-CYCODE-CVE-2026-59951 (GHSA-84M7-P3X7-PCFV)

Vulnerability from osv_homebrew – Published: 2026-10-16 08:52 – Updated: 2026-10-16 08:52 – Source website
VLAI
Summary
MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
Details

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.streamable_http_app() (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 stateless_http=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 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.

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
6.9 (Medium)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
References
To clipboard 

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "resource": "mcp",
        "resource_purl": "pkg:pypi/mcp@1.30.0",
        "upstream_fixed_in": "1.30.0"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "cycode",
        "purl": "pkg:brew/cycode"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.2.0"
            },
            {
              "fixed": "3.23.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/mcp@1.30.0",
        "name": "mcp",
        "resource": "mcp",
        "strategy": "registry",
        "subject_version": "1.30.0"
      }
    ]
  },
  "details": "### Summary\nIn affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state:\n\n- sessions did not expire;\n- a client `DELETE` ended the session but left its entry registered;\n- a session whose first request was refused stayed registered with a running task.\n\nEach 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.\n\n### Am I affected?\nYes, 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:\n\n- `FastMCP` (1.x)\n- `MCPServer` and `Server.streamable_http_app()` (2.x)\n- `StreamableHTTPSessionManager`\n\nAffected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1.\n\nNot affected:\n\n- servers on 1.12.0 or later passing `stateless_http=True` (no sessions are kept)\n- servers using only the stdio or SSE transports\n\nAuthentication limits who can open sessions, not how many a token holder can open.\n\n### Fix\nUpgrade to `mcp` 1.30.0 (1.x) or 2.2.0 (2.x) or later. In those versions:\n\n- 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;\n- at most `max_sessions` sessions exist at once (default 10000); a request that would open another is answered `503`;\n- a session is discarded as soon as it ends, whether by `DELETE`, expiry, or a refused opening request.\n\nBoth settings are keyword arguments on `FastMCP(...)` (1.x), `MCPServer.run()` / `streamable_http_app()` (2.x) and `StreamableHTTPSessionManager`; `None` disables them.\n\nThis 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.\n\nIf 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.",
  "id": "BREW-cycode-CVE-2026-59951",
  "modified": "2026-10-16T08:52:01Z",
  "published": "2026-10-16T08:52:01Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-59951"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/pull/3395"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/pull/3426"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932fc1d994e7"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e6575931b"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/python-sdk"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion",
  "upstream": [
    "GHSA-84m7-p3x7-pcfv",
    "CVE-2026-59951"
  ]
}



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