Georgia Tech students say they can pose as any user -- like a celebrity -- and send the one-word greeting to other, unwitting users. “We can get any Yo user’s phone number (I actually texted the founder, and he called me back.) We can spoof Yos from any users, and we can spam any user with as many Yos as we want. We could also send any Yo user a push notification with any text we want (though we decided not to do that.),” one of the hackers said.