CVE-2026-111769 is a medium-severity (CVSS 6.5) vulnerability in @modelcontextprotocol/sdk. A fix is available for @modelcontextprotocol/sdk — see the affected versions and patch details below.
MCP TypeScript client SDK: HTTP client transports and OAuth requests followed cross-origin redirects and re-sent custom headers and request bodies
Real-World Exposure
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.
@modelcontextprotocol/sdknpm@modelcontextprotocol/clientnpmDescription
Summary
In affected versions, the SDK's HTTP client transports and OAuth requests followed redirects to any origin.
If an MCP server or authorization server redirected a request to another origin, that origin received custom request headers (such as an X-API-Key set in requestInit.headers) and the mcp-session-id header. On a 307 or 308 it also received the request body, including OAuth token requests with their refresh_token and client_secret.
On current runtimes fetch already drops the Authorization header across origins.
Am I affected?
Yes, if your application uses StreamableHTTPClientTransport, SSEClientTransport, or the SDK's OAuth client functions such as auth(). stdio clients are not affected.
Affected versions:
@modelcontextprotocol/sdkall versions through 1.31.0@modelcontextprotocol/client2.0.0 through 2.2.0
Fix
Upgrade to:
- 1.x:
@modelcontextprotocol/sdk1.32.0 or later - 2.x:
@modelcontextprotocol/client2.3.0 or later
Redirects are now followed only within the same origin, and not at all in browsers. If your MCP server or authorization server redirects to another origin on purpose, configure the final URL instead. Setting redirectPolicy: 'follow' on a transport restores the old behaviour and the exposure.
If a redirect may have reached a host you do not control, rotate the API keys and client_secret your application held and revoke its tokens.
If you cannot upgrade yet, pass a fetch that refuses redirects, for example (url, init) => fetch(url, { ...init, redirect: 'error' }).
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @modelcontextprotocol/sdk | all versions | 1.32.0npm install @modelcontextprotocol/sdk@1.32.0 |
| 📦npm | @modelcontextprotocol/client | ≥ 2.0.0&&< 2.3.0 | 2.3.0npm install @modelcontextprotocol/client@2.3.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @modelcontextprotocol/sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @modelcontextprotocol/sdk to 1.32.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-111769 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
Is CVE-2026-111769 in your dependencies?
Find it across npm, including transitive dependencies.