The Microsoft Problem
"We write our cookies to multiple domains to give users a good experience with single sign-on,"
And that is the best explanation of the microsoft problem that you will ever see. Nearly all of their security problems stem from making things easy for users rather than making them secure.
I don't want single sign on across domains, I want it to be secure. Ideally I'd like to have different credentials on different domains. But I know there are plenty of fucktards out there who use the same credentials on every site they visit because it's too hard for the poor dears to remember more than one set of credentials. They never change the password and then get all upset when somebody manages to hack into several of their accounts.