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Advisory record · GHSA-84m7-p3x7-pcfv

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

CVSS 6.9 MediumEPSS 0.0046 · 39th percentile

Published by the GitHub Advisory Database at 15 Oct 2026, 20:13 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
mcpPyPI>=1.8.0, <1.30.0 || >=2.0.0, <2.2.01.30.0, 2.2.0
Identifiers
GHSA-84m7-p3x7-pcfv · CVE-2026-59951
Weakness (CWE)
CWE-400 · CWE-770 · CWE-772
Tags
Published
15 Oct 2026
Updated
15 Oct 2026

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Affected versions of the mcp Python SDK (1.8.0 through 1.29.1, and 2.0.0 through 2.1.1) run Streamable HTTP servers in stateful mode, which is the default, and never reclaim session state, so sessions accumulate without limit. A client that can reach the endpoint can use ordinary short requests to grow server memory until the process restarts. Upgrade to mcp 1.30.0 or 2.2.0 or later, which add idle expiry and a maximum session count; if you cannot upgrade, pass stateless_http=True where per-session state is not needed.

Written 1 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

ScoreRatingCVSSVectorSource
6.9 Medium 4.0 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 CNA (GitHub_M)
  • CISA KEV Not listed.
  • EPSS 0.0046, the 39th percentile, on 8 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 31 Oct 2026, 14:08 UTC First seen by agentprime
  2. 31 Oct 2026, 14:08 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.