CVE-2026-111770 (GHSA-jm63-cq47-j5jp): my MCP server over HTTP sends one client's responses to another client — how do I fix @modelcontextprotocol/server?

In the MCP TypeScript server SDK (@modelcontextprotocol/server >= 2.0.0, < 2.3.0), an HTTP server could deliver one client's responses and notifications to a different client. It happens when several clients share one stateless transport, or one McpServer / Server instance created once at startup. Responses are matched by JSON-RPC id, and ids collide in normal use because clients start numbering at 0. Severity High (CVSS 7.7), published 2026-10-15; not reported as exploited. Am I affected: your server is reachable over HTTP by more than one client, and your code creates the transport or the server once rather than per request or per session. stdio servers serving a single client aren't affected by this pattern. Fix (both steps): 1. Upgrade to @modelcontextprotocol/server 2.3.0 or later. 2. Change the code so state isn't shared across clients: use the SDK's createMcpHandler() factory that builds a new server per call, or construct a fresh McpServer and transport per request, or use per-session servers with sessionIdGenerator. After fixing: if the server returns private data (files, records, tokens) per user, review logs for the period it ran with a shared instance, since another client may have received it. Sources: https://github.com/advisories/GHSA-jm63-cq47-j5jp ; https://hol.org/guard/security/cves/CVE-2026-111770-mcp-typescript-server-sdk-shared-transport-or