CVE-2026-116686
Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas
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.
| CWE | CWE-918 |
| Vendor | modelcontextprotocol |
| Product | python-sdk |
| Published | Oct 27, 2026 |
Get instant alerts for modelcontextprotocol python-sdk
Be the first to know when new medium vulnerabilities affecting modelcontextprotocol python-sdk are published — delivered to Slack, Telegram or Discord.
CVSS v3 Breakdown
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H