You're overthinking it. There will be a rare issue
by dulac89 (2015-01-03 11:07:29)

In reply to: How do I validate those logins?  posted by El Kabong

of fraudulent login, but the convenience to your users (especially the ones that pay) far overrides that. This is no different than just about every other login site that I access.

Agree with the point that the first time login from a computer must be validated, but after that should be no big deal. If someone is using a public computer they should be smart enough to log out