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Back to active CVEs
Medium · CVSS 6.9CVE-2026-59951

MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource ExhaustionCVE-2026-59951

Answer in brief

CVE-2026-59951 records a Medium severity (CVSS 6.9) vulnerability in MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion. The current sources do not mark it as known exploited. The current feed maps modelcontextprotocol/python-sdk (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 14, 2026Updated Oct 14, 2026Source checked Oct 14, 2026First seen by HOL Oct 14, 2026Material review Oct 14, 2026
Upstream Advisory

Key facts

Risk
Medium · CVSS 6.9
Exploitation
Not marked as known exploited
Affected software
1 mapped package or product
Fix availability
Available

Why this deserves its current priority

CVSS is 6.9. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps modelcontextprotocol/python-sdk (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
modelcontextprotocol/python-sdkgeneric>=1.8.0 <1.30.0 || >=2.0.0 <2.2.01.30.0, 2.2.0

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for modelcontextprotocol/python-sdk.
  2. 2Review the reported fix. Update modelcontextprotocol/python-sdk to 1.30.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

  1. Published upstream

    Oct 14, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Oct 14, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Oct 14, 2026

Sources and claim methodology

  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
Upstream source description

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). From 1.8.0 until 1.30.0 and 2.2.0, stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager, retain session entries without an idle expiration or aggregate session-count limit. DELETE, refused opening requests, and completed sessions do not reliably discard their entries, and each entry can retain parsed initialize parameters. A client that can reach the endpoint can therefore create ordinary short requests that grow server memory without bound until restart. Stateless HTTP, stdio, and SSE transports are not affected. This issue is fixed in versions 1.30.0 and 2.2.0.

Quoted source text, attributed separately from HOL analysis.

Related CVEs

  • HTTP/2 blocked-send deadlines reset on every WINDOW_UPDATE, letting a trickle pin stream processes indefinitely in BanditAlso CWE-400
  • Junos OS and Junos OS Evolved: In a scenario with VPN-flowspec and BMP monitoring a specific BGP update causes an rpd crashSame generic ecosystem
  • Junos OS and Junos OS Evolved: A malformed LSP-ping causes an rpd crashSame generic ecosystem
  • Junos OS and Junos OS Evolved: A specific BGP packet causes an rpd crash when an ECMP qualifier is used in an import policySame generic ecosystem
  • Junos OS and Junos OS Evolved: An attacker sending a malformed update causes an rpd crash when BGP EVPN signaling is usedSame generic ecosystem
  • Junos OS and Junos OS Evolved: A BGP EVPN neighbor causes an rpd crash when SRv6 multihoming is usedSame generic ecosystem

Record context

Vulnerability class
Tool Poisoning
EPSS
Not reported
CWE IDs
CWE-400, CWE-772
Source
CVE List V5
Source checked
Oct 14, 2026
References
7 linked sources
Open source record