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High, CVSS 7.7amended

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

CVE-2026-111770GHSA-jm63-cq47-j5jp

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (@modelcontextprotocol/server 2.3.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
7.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:H
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-jm63-cq47-j5jp

  2. Revision 2 · amended ·

    Amended from osv record GHSA-jm63-cq47-j5jp

  3. Revision 1 · created ·

    Published from ghsa record GHSA-jm63-cq47-j5jp

Agent brief

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# Patch Next advisory brief: CVE-2026-111770

Advisory: MCP TypeScript server SDK: Shared transport or server instances delivered one HTTP client's messages to another
IDs: CVE-2026-111770, GHSA-jm63-cq47-j5jp
Record: https://patchnext.com/advisories/cve-2026-111770 (revision 3, source checked 26 Oct 2026, 13:07 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @modelcontextprotocol/server 2.3.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @modelcontextprotocol/server: affected >= 2.0.0, < 2.3.0; fixed in 2.3.0

## Suggested inspection (read-only)
- `npm ls @modelcontextprotocol/server --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @modelcontextprotocol/server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-jm63-cq47-j5jp (GitHub-reviewed advisory GHSA-jm63-cq47-j5jp lists affected ranges: npm @modelcontextprotocol/server >=2.0.0 <2.3.0)
- Verified: https://osv.dev/vulnerability/GHSA-jm63-cq47-j5jp (OSV record GHSA-jm63-cq47-j5jp lists affected ranges: npm @modelcontextprotocol/server >=2.0.0 <2.3.0 (fixed))

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