CVE-2026-59951 (GCVE-0-2026-59951)

Vulnerability from cvelistv5 – Published: 2026-10-14 16:22 – Updated: 2026-10-15 15:07
VLAI
Title
MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
Summary
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.
Severity
6.9 (Medium)
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
SSVC
Exploitation: none Automatable: yes Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-10-15 13:55 UTC
CWE
  • CWE-400 - Uncontrolled Resource Consumption
  • CWE-772 - Missing Release of Resource after Effective Lifetime
Assigner
GitHub_M CNA under the mitre root
CNA scorecard C 72/100 over 7771 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
modelcontextprotocol python-sdk Affected: >= 1.8.0, < 1.30.0
Affected: >= 2.0.0, < 2.2.0
guessed Create a notification for this product.
Show details on NVD website
To clipboard 

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