When you deactivate all third party cookies in firefox, log in to gmail and then visit youtube.com in a new tab, you are automatically signed in. How does google achieve that? I have checked for 302 redirects using the persistent log option, but I do not see any. All other questions about this topic I have found are really outdated, so hopefully somebody can explain how google has implemented this.
1 Answers
A third-party cookie limit setting cookies on different domains than where the user actually is. If you are visiting mail.com and it fetches something from video.com, the cookies from that domain are automatically sent by setting the credentials to include. However, if third-party cookies are disabled, those cookies are not sent because the browser only allows cookies from mail.com.
What Google actually does is when you log in, it does a few redirect to save cookies on mail.google.com as well as on youtube.com. If you open the DevTools and check the Preserve log option, you will clearly see those redirection. They are done so quick that you don't see them but it is how you can be logged in into multiple domains at the same time.
See the redirections you get from logging into your Google account
By doing those redirection, Google makes sure to save the cookies on the Google domain as well as the YouTube domain as way. This is why even though your third-party cookies are disabled, you get logged in into multiple domains.
Related