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HIGH severity

MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource ExhaustionGHSA-84m7-p3x7-pcfv

HIGHFix: modelcontextprotocol/python-sdk#3395

GHSA-84m7-p3x7-pcfv is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in mcp. A fix is available for mcp — see the affected versions and patch details below.

Also known asCVE-2026-59951
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Oct 22, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-84m7-p3x7-pcfv.

EPSS Exploitation Probability

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs39th percentile — riskier than 39% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-84m7-p3x7-pcfv by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 394,415 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected
🐍mcp🐍mcp

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImcp≥ 1.8.0&&< 1.30.01.30.0pip install --upgrade 'mcp==1.30.0'
🐍PyPImcp≥ 2.0.0&&< 2.2.02.2.0pip install --upgrade 'mcp==2.2.0'

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mcp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update mcp to 1.30.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-84m7-p3x7-pcfv is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### 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 H
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