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CVE-2026-59951

CVE-2026-59951 · MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion

This page contains facts from public vulnerability sources. Check the original advisory before updating.

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.

CVSS 6.9 · Medium · EPSS 0.5% · Fix available

Published
Updated

Stack: python

Affected products and versions

  • python-sdk

    Versions
    • >= 1.8.0, < 1.30.0: Affected
    • >= 2.0.0, < 2.2.0: Affected

    No fix published

  • mcp PyPI

    Versions
    • 1.10.0
    • 1.10.1
    • 1.11.0
    • 1.12.0
    • 1.12.1
    • 1.12.2
    • 1.12.3
    • 1.12.4
    • 1.13.0
    • 1.13.1
    • 1.14.0
    • 1.14.1
    • 1.15.0
    • 1.16.0
    • 1.17.0
    • 1.18.0
    • 1.19.0
    • 1.20.0
    • 1.21.0
    • 1.21.1
    • 1.21.2
    • 1.22.0
    • 1.23.0
    • 1.23.1
    • 1.23.2
    • 1.23.3
    • 1.24.0
    • 1.25.0
    • 1.26.0
    • 1.27.0
    • 1.27.1
    • 1.27.2
    • 1.28.0
    • 1.28.1
    • 1.29.0
    • 1.29.1
    • 1.8.0
    • 1.8.1
    • 1.9.0
    • 1.9.1
    • 1.9.2
    • 1.9.3
    • 1.9.4
    Affected ranges
    • ECOSYSTEM: ≥ 1.8.0, < 1.30.0

    Update to 1.30.0

  • mcp PyPI

    Versions
    • 2.0.0
    • 2.0.1
    • 2.1.0
    • 2.1.1
    Affected ranges
    • ECOSYSTEM: ≥ 2.0.0, < 2.2.0

    Update to 2.2.0

Original sources

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