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

Quick assessment

Affected
modelcontextprotocol python-sdk
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

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 StreamableHTTPSessionMa

CVSS 6.9 · Medium

Affected Version Matrix 2

VendorProduct Version RangeStatus
modelcontextprotocol python-sdk >= 1.8.0, < 1.30.0 affected
>= 2.0.0, < 2.2.0 affected
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I. Basic Information for CVE-2026-59951

Vulnerability Information

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Vulnerability Title
MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
未加控制的资源消耗(资源穷尽)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
modelcontextprotocol python-sdk >= 1.8.0, < 1.30.0 -

II. Public POCs for CVE-2026-59951

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III. Intelligence Information for CVE-2026-59951

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Patches & Fixes for CVE-2026-59951 (4)

Other References for CVE-2026-59951 (3)

Same Patch Batch · modelcontextprotocol · 2026-10-14 · 3 CVEs total

CVE-2026-108395 7.5 HIGH Model Context Protocol Java SDK HTTP server transports allow memory exhaustion through unb
CVE-2026-109527 7.5 HIGH Model Context Protocol Python SDK HTTP and OAuth endpoints allow memory exhaustion through

IV. Related Vulnerabilities

Same product: python-sdk
Same vendor: modelcontextprotocol
Same weakness: CWE-400

V. Comments for CVE-2026-59951

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