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/sdk1.26.0 and later
What to do
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 newMcpServeron every call.- Per request: build a new
McpServerand a new transport inside the request handler. - Per session: set
sessionIdGeneratorand keep oneMcpServerand one transport per session.
If you cannot upgrade yet, the same change fixes affected versions.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | @modelcontextprotocol/server | >= 2.0.0, < 2.3.0 | 2.3.0 |
References
- https://github.com/modelcontextprotocol/typescript-sdk/security/advisories/GHSA-jm63-cq47-j5jp
- https://github.com/modelcontextprotocol/typescript-sdk/pull/2918
- https://github.com/modelcontextprotocol/typescript-sdk/commit/84804c22e45a662675a198f853b7f00063838a8d
- https://github.com/modelcontextprotocol/typescript-sdk
- CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:H- Weakness (CWE)
CWE-362- Platform
- npm
- Package
@modelcontextprotocol/server- Disclosed
- 20.10.2026
- Updated
- 20.10.2026 23:19
Sources and licenses · Edited or translated by DockRay; source rights and applicable licenses are retained.
OSV / GitHub Advisory Database - GHSA-jm63-cq47-j5jp - CC BY 4.0
Vulnerability history: @modelcontextprotocol/server
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 3
- First → latest
- 10.2026 10.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 2
- Medium 1
Disclosures per year
- 2026: 3
-
MCP TypeScript server SDK: Shared transport or server instances delivered one HTTP client's messages to another this entry
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