text: Creator Or Arbel told technology news site TechCrunch the app was having "security issues".
  The app allows users to send a message saying "yo" to friends - and nothing else.
  It has been branded "pointless", but has nonetheless raised $1m (Â£600,000) in investment.
  TechCrunch said it was contacted by three college students who said they had uncovered a flaw in the app.
  "We can get any Yo user's phone number (I actually texted the founder, and he called me back)," the students told TechCrunch.
  "We can spoof yos from any users, and we can spam any user... We could also send any Yo user a push notification with any text we want (though we decided not to do that)."
  Other developers have been able to recreate the flaw.
  Similar problems have hit apps such as Snapchat and Tinder in the past few months.
  Mr Arbel told TechCrunch he was dealing with the issue, but would not elaborate further.
  "Some of the stuff has been fixed and some we are still working on," he said, adding that he had hired a specialist security team.
  "We are taking this very seriously."
  Despite the app's apparent pointlessness, it has quickly attracted hundreds of thousands of users. Mr Arbel said more than four million "yo" messages had been sent.
  A service that sends a "yo" to a user every time a goal is scored in the World Cup has already been launched - with other similar services planned.
  The app, which took eight hours to create, has the technology community divided - with some seeing the app, and its seven-figure investment, as a sign of increasing hysteria in Silicon Valley for offbeat ideas.
  "We have decided this is an idea with great potential," Mr Arbel told the Financial Times.
  One reader of The Verge technology news site said the app, and its sudden mainstream coverage, was "accelerating the decline of civilisation".
  Follow Dave Lee on Twitter @DaveLeeBBC
target: Messaging app Yo, which in the past week has rocketed to the top of the app download charts, has been hit by a hack.