[nodamage]

Yes. If you've set up your Slack so each login checks against the identity provider to ensure an active user is logging in, that would resolve the issue, no?

Even if you take over company.com's domain you can't reconfigure company.com's Slack to point to a new identity provider?

[radlad]

I think you may be a bit confused about the players here. When you use Google OAuth to login, it _is_ your identity provider, and it is reporting to Slack that the user exists. Google is reporting the user exists because it exists in the Google Workspace directory. You use this as your source of truth for provisioning users, and they automatically get access to all of your company's apps.

The problem is that even though the user has the same email ([e-mail]), and the same Google Workspace domain ("hd": example.com), this is actually a _new_ Google Workspace account. But nothing Google provides to Slack allows them to detect this.

Slack, et al can fix this by _not_ using the public Google OAuth integration, and forcing every use to configure an individual internal Google OAuth integration. But they use the public one because Google has said it is a safe and secure way to operate their service.

[nodamage]

What I'm suggesting is if you were able to pre-configure Slack to only allow logins for valid users from Google Workspace X, then even if someone creates a new workspace Y with the same domain, Slack would still be checking against workspace X. (And similar for non-Google based identity providers.)

[tsimionescu]

And people are telling you that this is not possible with the Google public OAuth API. When Slack asks Google's public OAuth instance if user [e-mail] is valid, Google checks with the Google Workspace associated to example.com, and returns to Slack a response saying "yes, that user is valid, here is more information from example.com". This can be the same Workspace or another one, Google isn't really telling Slack apparently.

Now, there is another field called "sub", that should be a unique ID for the Google Workspace or the specific account, but it seems that this is not always stable, per the article, so people integrating with Google OAuth don't trust it.

[nodamage]

> And people are telling you that this is not possible with the Google public OAuth API.

Yes I understand, however it is possible to integrate Slack and Google SSO in such a way that it checks that the user belongs to the correct workspace, correct? Either via the SAML integration (https://support.google.com/a/answer/6357481) or an internal Google OAuth integration? The purpose of the public Google OAuth API as opposed to the previous two options is to allow logins from non-workspace or cross-workspace Google accounts, correct?

[SAI_Peregrinus]

No, there's no way to check which workspace is in use, just that the domain matches. That's the problem.

[nodamage]

Only if you use the Google public OAuth integration. If you instead use the SAML integration with Slack as described in the link above you don’t have this problem.