@modelcontextprotocol/sdk has cross-client data leak via shared server/transport instance reuse

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-25536: 0.29% estimated exploitation activity in the next 30 days; percentile 22.00 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Cross-client data leak via two distinct issues: (1) reusing a single StreamableHTTPServerTransport across multiple client requests, and (2) reusing a single McpServer/Server instance across multiple transports. Both are most common in stateless deployments. Impact This advisory covers two related but distinct vulnerabilities. A deployment may be affected by one or both. Issue 1: Transport re-use What happens: When a single StreamableHTTPServerTransport instance handles multiple client requests, JSON-RPC message ID collisions cause responses to be routed to the wrong client's HTTP connection. The transport maintains an internal requestId → stream mapping, and since MCP client SDKs generate message IDs using an incrementing counter starting at 0, two clients produce identical IDs. The second client's request overwrites the first client's mapping entry, routing the response to the wrong HTTP stream. What is affected: All request types — tools/call, resources/read, prompts/get,…

Published by the source

4 Feb 2026, 20:04 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 12:42 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-345p-7cg4-v4c7

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-345p-7cg4-v4c7

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-25536

Advisory: @modelcontextprotocol/sdk has cross-client data leak via shared server/transport instance reuse
IDs: CVE-2026-25536, GHSA-345p-7cg4-v4c7
Record: https://patchnext.com/advisories/cve-2026-25536 (revision 2, source checked 26 Oct 2026, 12:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @modelcontextprotocol/sdk 1.26.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @modelcontextprotocol/sdk: affected >= 1.10.0, <= 1.25.3; fixed in 1.26.0

## Suggested inspection (read-only)
- `npm ls @modelcontextprotocol/sdk --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/sdk.
- 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://osv.dev/vulnerability/GHSA-345p-7cg4-v4c7 (OSV record GHSA-345p-7cg4-v4c7 lists affected ranges: npm @modelcontextprotocol/sdk >=1.10.0 <1.26.0 (fixed))
- Verified: https://github.com/advisories/GHSA-345p-7cg4-v4c7 (GitHub-reviewed advisory GHSA-345p-7cg4-v4c7 lists affected ranges: npm @modelcontextprotocol/sdk >=1.10.0 <=1.25.3)

Commands are suggestions: review each one before running it.