Since you seem to work for Microsoft, the broken use case I know: without 3p cookies Microsoft Teams is broken (it shows a not very useful error page asking to refresh the page, when you do it shows the same error again). Many of us need it for remote work so I guess this one of the things Google had in mind when they delayed disabling 3p cookies by default.

Yes, Teams relies on silent auth from within an iframe (all those chat windows etc). This is impossible without 3p cookies, so we're working with Google to find a solution. Storage access API in Safari works, but sounds like it won't make it to Chrome.

Thanks, nice to know you are working on it.

Auth should be doable with just redirects though right? Isn't that how OAuth and OpenID connect work?

Same thought. I’m curious to know what other kinds of authentication protocols require third-party cookies to operate. Within OIDC, even more obscure/advanced features such as session management and global logout require only 1st party (the IdPs) cookies in order to function.

I guess the tradeoff being made here is just leaning into our reliance on the certificate authority system. Whereas before, with third party cookies, you might have had more flexibility with how you structure your domains.

The implicit grant, while deprecated, is still used across the ecosystem. Further, embedded apps (eg you have a portal that iframes Salesforce and Salesforce needs to be authenticated) can't redirect to the login page or open that iframe for cookies.

Front channel logout is also broken, part of the OIDC spec. It opens iframes to sites, but the sites don't get their cookies and can't write them.

OAuth sometimes is done using an iframe and a pop-up window instead of by navigating to the federated auth provider and back. That version often needs third party cookies.