Comment by nodamage

Google's public OAuth is intended for situations where you want to allow anyone with a Google account to login to your application, regardless of workspace. Given that, it is not a Google OAuth vulnerability that a user can login to your application using a Google account from a different workspace.

If you want to integrate Google SSO into your application and restrict logins to a specific workspace, there are other options you can use that will actually check if the user belongs to that workspace (SAML or internal OAuth).

To the extent that service providers are using Google's public OAuth and then trying to read the domain name out of the returned ID token in order to restrict logins a specific Google workspace, they are using Google's public OAuth instance for a situation it was not intended for because domain names do not map 1:1 to workspaces. However that is a vulnerability on the service provider's side, not Google's.

To the extent that a service provider offers Google SSO integration via Google's public OAuth but also via workspace restricted options, if a company selects the former instead of the latter then the responsibility for the vulnerability is on the company's side. (Slack, for example, offers both because there are some Slack groups where allowing access from any Google account makes sense, and other groups where you would want to restrict access to Google accounts from a specific workspace.)