Summary
MCP TypeScript server SDK: Shared transport or server instances delivered one HTTP client's messages to another
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
Impact
Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing. Typical impact: TOCTOU exploits, data corruption, or privilege escalation.
CVE-2026-111770 has a CVSS score of 7.7 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.3.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
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.
Frequently Asked Questions
- What is CVE-2026-111770? CVE-2026-111770 is a high-severity race condition vulnerability in @modelcontextprotocol/server (npm), affecting versions >= 2.0.0, < 2.3.0. It is fixed in 2.3.0. Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing.
- How severe is CVE-2026-111770? CVE-2026-111770 has a CVSS score of 7.7 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of @modelcontextprotocol/server are affected by CVE-2026-111770? @modelcontextprotocol/server (npm) versions >= 2.0.0, < 2.3.0 is affected.
- Is there a fix for CVE-2026-111770? Yes. CVE-2026-111770 is fixed in 2.3.0. Upgrade to this version or later.
- Is CVE-2026-111770 exploitable, and should I be worried? Whether CVE-2026-111770 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-111770 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-111770? Upgrade
@modelcontextprotocol/serverto 2.3.0 or later.