---
id: CVE-2026-116686
title: >-
  Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs
  while validating tool results against output schemas
summary: >-
  The Model Context Protocol Python SDK is the official Python SDK for Model
  Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0,
  ClientSession, ClientSessionGroup, and Client tool calls validate
  server-declared output…
severity: medium
cvss: 6.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H'
cvssSource: cna
cwe:
  - CWE-918
vendor: modelcontextprotocol
product: python-sdk
affected:
  - 'python-sdk >= 1.10.0, < 1.30.0'
  - 'python-sdk >= 2.0.0, < 2.2.0'
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-10-28T14:34:36.544931Z'
published: '2026-10-27'
updated: '2026-10-28'
sourceUpdated: '2026-10-28T16:13:45.473Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-116686'
references:
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-rwrf-2pqf-9j8j
    label: >-
      https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-rwrf-2pqf-9j8j
  - url: 'https://github.com/modelcontextprotocol/python-sdk/pull/3394'
    label: 'https://github.com/modelcontextprotocol/python-sdk/pull/3394'
  - url: 'https://github.com/modelcontextprotocol/python-sdk/pull/3396'
    label: 'https://github.com/modelcontextprotocol/python-sdk/pull/3396'
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/commit/5ebdfed4060d8e5b11b8e2348f86bad4d1fb8836
    label: >-
      https://github.com/modelcontextprotocol/python-sdk/commit/5ebdfed4060d8e5b11b8e2348f86bad4d1fb8836
  - url: >-
      https://github.com/modelcontextprotocol/python-sdk/commit/6705402e246dc4eb1fcdf4d9902b78d3c9c36e1b
    label: >-
      https://github.com/modelcontextprotocol/python-sdk/commit/6705402e246dc4eb1fcdf4d9902b78d3c9c36e1b
  - url: 'https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0'
    label: 'https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0'
  - url: 'https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0'
    label: 'https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0'
  - url: 'https://github.com/advisories/GHSA-rwrf-2pqf-9j8j'
tags:
  - cve.org
  - ghsa
  - pip
epss: 0.00337
epssPercentile: 0.27008
aliases:
  - GHSA-rwrf-2pqf-9j8j
ecosystem: pip
patched:
  - mcp 2.2.0
  - mcp 1.30.0
ingestedAt: '2026-10-27T19:54:27.804Z'
---

## Overview

The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.

## Affected

- `python-sdk >= 1.10.0, < 1.30.0`
- `python-sdk >= 2.0.0, < 2.2.0`

## Remediation

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

## Package advisory (CVE-2026-116686)

Affected packages:

- `mcp >= 2.0.0, < 2.2.0`
- `mcp >= 1.10.0, < 1.30.0`

Patched in:

- `mcp 2.2.0`
- `mcp 1.30.0`

Source: https://github.com/advisories/GHSA-rwrf-2pqf-9j8j