How severe is CVE-2026-111770?

CVE-2026-111770 has a CVSS score of 7.7/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-111770?

CVE-2026-111770 affects the following packages: @modelcontextprotocol/server (npm). Ecosystems affected: npm.

How do I fix CVE-2026-111770?

Update @modelcontextprotocol/server to 2.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-111770 is resolved across your whole dependency graph.

How do I detect CVE-2026-111770 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @modelcontextprotocol/server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-111770 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-111770 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-111770 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was CVE-2026-111770 published, and has it been updated?

CVE-2026-111770 was published on October 20, 2026 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

HIGH severity

CVE-2026-111770 — @modelcontextprotocol/ser…

HIGHFix: modelcontextprotocol/typescript-sdk#2918

CVE-2026-111770 is a high-severity (CVSS 7.7) vulnerability in @modelcontextprotocol/server. A fix is available for @modelcontextprotocol/server — see the affected versions and patch details below.

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

Published

Oct 20, 2026

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 20, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

3Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream

@modelcontextprotocol/servernpm

8.2Mdownloads / week

Description

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`@modelcontextprotocol/server`	≥ 2.0.0&&< 2.3.0	2.3.0`npm install @modelcontextprotocol/server@2.3.0`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @modelcontextprotocol/server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @modelcontextprotocol/server to 2.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-111770 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### 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

O3 Security · Impact-Aware SCA

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