The vulnerability in Google mcp-toolbox 1.4.0 involves improper authentication due to missing audience or clientId configuration in the Google OAuth provider component. When mcpEnabled is true and no audience/clientId is specified, the ValidateMCPAuth pipeline does not validate the token audience, resulting in acceptance of any valid Google OAuth access token regardless of its intended audience. This leads to an authentication bypass where unauthorized clients can access protected resources.

Unauthorized clients can gain access to protected tools and data backends by presenting valid Google OAuth access tokens minted for unrelated ecosystem applications. This bypasses intended audience restrictions and compromises the authentication mechanism, potentially exposing sensitive data or functionality.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, ensure that the Google OAuth provider component is configured with an explicit audience or clientId when mcpEnabled is true to enforce proper token audience validation.