GHSA-RWRF-2PQF-9J8J Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas

Summary In affected versions, when a connected MCP server declared an outputSchema for a tool, the SDK's client validated each calltool result with jsonschema's default resolver, which fetches any $ref it cannot resolve locally by opening the URL. A server could therefore put an http://, https://...