The vulnerability can be attacked without user login. Tests were performed on Firefox (26.0) in Ubuntu (12.04) and IE (9.0.15) in Windows 7.
(1) When a user is redirected from Google to another site, Google will check whether the redirected URL belongs to domains in Google’s whitelist (The whitelist usually contains websites belong to Google), e.g.
However, if the URLs in a redirected domain have open URL redirection vulnerabilities themselves, a user could be redirected from Google to a vulnerable URL in that domain first and later be redirected from this vulnerable site to a malicious site. This is as if being redirected from Google directly.