SOP
Consider a scenario where a target is logged into their Gmail account (mail.google.com) in one of the open browser tabs. In another tab, they navigate to a different site, on a different domain, which contains attacker code that wants access to that Gmail data. Maybe they were socially engineered to visit this particular site or maybe they were redirected there through a malicious advertising (malvertising) campaign on a well-known news site.
The attacker code may try to open a connection to the mail.google.com domain, and because the victim is already authenticated in the other browser tab, the code should be able to read and send emails as well by forging requests to Gmail. JavaScript provides all the tools necessary to accomplish all of this, so why isn't everything on fire?
The answer, as we will see in detail shortly, is because of the SOP. The SOP prevents this exact attack and, unless the attacker can inject their code directly into mail.google.com, they will not be able to read...
