javascript - How can a visitor logged into domain A be spared having to re-log in wherever domain A's widget is installed on other domains? -

scenario: mr. smith joins site foo , logs in. site bar, on different domain, has installed widget. mr. smith visits site bar , permitted use widget if member of site foo.

since many sites can install widget, how can mr. smith spared trouble of re-logging in wherever widget installed, if logged site foo?