For some reason this only just now occurred to me: What’s to stop some web site from carefully crafting an imitation of the Google “you need to sign in again” UI, storing your Google password, and storing from the other side the auth cookie from Google, so that it can then poke around through 100% of your Google content including any other site you’ve signed into with the same SSO login?
This is such a fundamental flaw in the whole concept that it’s obviously occurred to people and they’ve had time to come up with something to prevent it, but I can’t see how you could prevent it. Have I missed something? You might have a non-Google URL in the address bar during the faked sign-in, or you could use varying degrees of deception to attempt to make the address bar look legit, but I’d honestly be surprised if more than 20% of people even check the address bar every time they sign in to SSO. I don’t.
So what’s to make this not work?
My account uses 2FA and my browser remembers my password. If I get a sign in prompt but my browser doesn’t / won’t auto fill my password, I assume it knows something I don’t and I’m immediately suspicious.
Your scenario is absolutely valid and would probably catch a lot of people who don’t think about security.