CVE-2026-111771 is a medium-severity (CVSS 6.8) vulnerability in @modelcontextprotocol/sdk. A fix is available for @modelcontextprotocol/sdk — see the affected versions and patch details below.
MCP TypeScript SDK: Server bearer authentication accepted tokens issued for another service
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/servernpm@modelcontextprotocol/expressnpmDescription
Summary
In affected versions, the SDK's server-side bearer authentication had no way to check that a token was issued for your server (its audience). Where one authorization server issues tokens for several services, a valid token for another service was also accepted by your MCP server.
Am I affected?
Yes, if your HTTP server uses requireBearerAuth() or verifyBearerToken(), its authorization server also issues tokens for other services, and your verifyAccessToken() does not check the token's audience (aud) itself.
Affected versions:
@modelcontextprotocol/sdk1.6.0 through 1.31.0@modelcontextprotocol/server2.0.0 through 2.2.0@modelcontextprotocol/express2.0.0 through 2.0.1@modelcontextprotocol/server-legacy2.0.0 through 2.3.0
Fix
Upgrade to:
- 1.x:
@modelcontextprotocol/sdk1.32.0 or later - 2.x:
@modelcontextprotocol/server2.3.0 or later, together with@modelcontextprotocol/express2.0.2 or later if you use it @modelcontextprotocol/server-legacy2.3.1 or later
Upgrading alone changes nothing. The patched versions add an expectedResource setting that is off by default. To turn the check on, make both of these changes together:
- Pass the audience your authorization server puts in tokens for your server:
requireBearerAuth({ verifier, expectedResource: new URL('https://mcp.example.com/mcp') }). - Return the token's audience from
verifyAccessToken()asAuthInfo.resource, also aURL.
A token for another service then gets 401. With only the first change, every request gets 401.
If you cannot upgrade yet, reject any token whose aud does not include your server inside verifyAccessToken(). That closes the issue on every version.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @modelcontextprotocol/sdk | ≥ 1.6.0&&< 1.32.0 | 1.32.0npm install @modelcontextprotocol/sdk@1.32.0 |
| 📦npm | @modelcontextprotocol/server | ≥ 2.0.0&&< 2.3.0 | 2.3.0npm install @modelcontextprotocol/server@2.3.0 |
| 📦npm | @modelcontextprotocol/express | ≥ 2.0.0&&< 2.0.2 | 2.0.2npm install @modelcontextprotocol/express@2.0.2 |
| 📦npm | @modelcontextprotocol/server-legacy | ≥ 2.0.0&&< 2.3.1 | 2.3.1npm install @modelcontextprotocol/server-legacy@2.3.1 |
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-2026-111771 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-2026-111771 in your dependencies?
Find it across npm, including transitive dependencies.