The only aspect I'll take issue with is that in many (granted not all, but many) OAuth use-cases, the user is required to re-login to a site -- even if it was a

The only aspect I'll take issue with is that in many (granted not all, but many) OAuth use-cases, the user is required to re-login to a site -- even if it was already authorized. The length of the cookie/token for an OAuth login varies, but more and more, I see very few services that don't require me to login again -- or at least manually click the "login" button. So for step 3 to happen, Mallory needs to try to target a site that Bob is currently logged into or that has a session life that is still active. Otherwise, Bob still sees his Twitter login logo and he has to make a choice to click or retreat.