A child frame can call top.focus() even if the framing page comes from a different
origin and has overridden the focus() routine. The call is made in the context
of the child frame. The attacker would look for a target site with a framed
page that makes this call but doesn't verify that its parent comes from the same
site. The attacker could steal cookies and passwords from the framed page, or
take actions on behalf of a signed-in user. This attack would work only against
sites that use frames in this manner.