Imagine a scammer can easily spoof someone into entering their details - also with so much ease - from what I have read/seen I feel there is no obvious robustness in this particular system. If this reaches masses, you can imagine Credit Card scam can actually be possible that much more easily.
Again, I have to say its way too early to say anything about this service. I just hope Square Up guys make it more obvious as to the security they provide to the End-User.
