CVE-2026-116686

Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas

Published Oct 27, 2026 CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
Description
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-rwrf-2pqf-9j8j Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas
History
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Modelcontextprotocol
Modelcontextprotocol python-sdk
Vendors & Products Modelcontextprotocol
Modelcontextprotocol python-sdk
Type Values Removed Values Added
Description 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.
Title Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas
Weaknesses CWE-918
References
Metrics cvssV3_1

{'score': 6.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Oct 28, 2026, 4:13 PM
Reserved
Oct 26, 2026, 8:00 PM
Status
Deferred
Published
Oct 27, 2026, 8:17 PM
Modified
Oct 28, 2026, 5:18 PM
Severity
Moderate
Public date
Oct 27, 2026, 7:29 PM