CVE-2026-111771: Medium severity npm/@modelcontextprotocol/server-legacy vulnerability

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/sdk 1.6.0 through 1.31.0 - @modelcontextprotocol/server 2.0.0 through 2.2.0 - @modelcontextprotocol/express 2.0.0 through 2.0.1 - @modelcontextprotocol/server-legacy 2.0.0 through 2.3.0

Fix Upgrade to:

- 1.x: @modelcontextprotocol/sdk 1.32.0 or later - 2.x: @modelcontextprotocol/server 2.3.0 or later, together with @modelcontextprotocol/express 2.0.2 or later if you use it - @modelcontextprotocol/server-legacy 2.3.1 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:

1. Pass the audience your authorization server puts in tokens for your server: requireBearerAuth({ verifier, expectedResource: new URL('https://mcp.example.com/mcp') }). 2. Return the token's audience from verifyAccessToken() as AuthInfo.resource, also a URL.

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.