---
id: CVE-2026-59951
title: >-
  The MCP Python SDK, called mcp on PyPI, is a Python implementation of the
  Model Context Protocol (MCP)
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 …
severity: none
cwe:
  - CWE-400
  - CWE-772
published: '2026-10-14'
updated: '2026-10-14'
sourceUpdated: '2026-10-14T17:18:29.690'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-59951'
references:
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/commit/92120b4b0f50385af096592d7cee932fc1d994e7
    label: security-advisories@github.com
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/commit/d2290ca3434731b68ea3e2270bc06a6e6575931b
    label: security-advisories@github.com
  - url: 'https://github.com/modelcontextprotocol/python-sdk/pull/3395'
    label: security-advisories@github.com
  - url: 'https://github.com/modelcontextprotocol/python-sdk/pull/3426'
    label: security-advisories@github.com
  - url: 'https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0'
    label: security-advisories@github.com
  - url: 'https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0'
    label: security-advisories@github.com
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-84m7-p3x7-pcfv
    label: security-advisories@github.com
tags:
  - nvd
ingestedAt: '2026-10-14T17:44:53.676Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.