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Medium, CVSS 6.9amended

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

CVE-2026-59951GHSA-84m7-p3x7-pcfv

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (mcp 2.2.0, 1.30.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI mcp

    Affected
    >= 1.8.0, < 1.30.0>= 2.0.0, < 2.2.0
    Fixed in
    2.2.0, 1.30.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.9 · 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 · score source
EPSS
  • CVE-2026-59951: 0.46% estimated exploitation activity in the next 30 days; percentile 39.75 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 4 · amended ·

    Amended from ghsa record GHSA-84m7-p3x7-pcfv

  2. Revision 3 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 2 · amended ·

    Amended from osv record GHSA-84m7-p3x7-pcfv

  4. Revision 1 · created ·

    Published from ghsa record GHSA-84m7-p3x7-pcfv

Agent brief

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# Patch Next advisory brief: CVE-2026-59951

Advisory: MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
IDs: CVE-2026-59951, GHSA-84m7-p3x7-pcfv
Record: https://patchnext.com/advisories/cve-2026-59951 (revision 4, source checked 26 Oct 2026, 13:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp 2.2.0, 1.30.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mcp: affected >= 1.8.0, < 1.30.0; >= 2.0.0, < 2.2.0; fixed in 2.2.0, 1.30.0

## Suggested inspection (read-only)
- `python -m pip show mcp`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-84m7-p3x7-pcfv (GitHub-reviewed advisory GHSA-84m7-p3x7-pcfv lists affected ranges: PyPI mcp >=1.8.0 =2.0.0 <2.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-84m7-p3x7-pcfv (OSV record GHSA-84m7-p3x7-pcfv lists affected ranges: PyPI mcp >=1.8.0 =2.0.0 <2.2.0 (fixed))

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