MCP TypeScript server SDK: Shared transport or server instances delivered one HTTP client's messages to another

### Summary In affected versions, an MCP server reached over HTTP could deliver one client's responses and notifications to another client. This happened when clients shared one stateless transport, or one `McpServer` or `Server` instance. Messages were matched to a connection by JSON-RPC `id` alone. SDK clients number requests from 0, so ids collide in normal use. ### Am I affected? Yes, if more than one client can reach your HTTP endpoint and your code creates the transport or the server once at startup, instead of once per request or per session. Until this release, the README examples of `@modelcontextprotocol/hono`, `express`, `fastify` and `node` showed that pattern. Affected versions: `@modelcontextprotocol/server` 2.0.0 through 2.2.0. Not affected: - `createMcpHandler()` with a factory that builds a new server on every call - stdio servers - `@modelcontextprotocol/sdk` 1.26.0 and later ### Fix Upgrade to `@modelcontextprotocol/server` 2.3.0 or later. **You also need to change your code.** The SDK now refuses to be shared, so an affected app fails requests after upgrading: a second `connect()` rejects with `ALREADY_CONNECTED`, and a stateless transport rejects its second request. Change to one of these: - **`createMcpHandler()`:** pass a factory that builds a new `McpServer` on every call. - **Per request:** build a new `McpServer` and a new transport inside the request handler. - **Per session:** set `sessionIdGenerator` and keep one `McpServer` and one transport per session. If you cannot upgrade yet, the same change fixes affected versions.

Source published

20 Oct 2026 20:07:29 UTC

Source modified

20 Oct 2026 20:15:05 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:H

Publisher rating · HIGH