The messaging app Yo, which lets users to send simple "yo" messages to contacts and which has seen a dramatic rise to the top of app downloads in recent weeks, has been attacked by hackers.

The company's creator Or Arbel told reporters they were having "security issues."

It follows much media attention for the simple app, which the creators believe will help streamline and simplify communications as it allows users to simply give a shout out to a colleague or friend. One of the main reasons the app was created was to allow users to send a quick message, with no frills, to a colleague to come into the room for a meeting or to discuss something.

But now, it's been hacked, a move that raises concerns over the security of the app. Students, it appears, have uncovered the flaw in the app. While many believe it to be not worth any time, it has garnered some $1 million in investments as it skyrocketed into popularity.

"We can get any Yo user's phone number (I actually texted the founder, and he called me back)," the students said in a news report.

"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)."

According to Moshe Hogeg, CEO of Mobli, which is a photo and video sharing service, Hogeg became frustrated by having to repeatedly call and text his assistant when he wanted her to come into his office and so developed the "yo" idea to streamline reaching out. It uses the idea that you can communicate without being forced to deliver long-winded messages.

It took some eight hours to create the app and the coding, showing that even the simplest app can be a massive success.

The app has roughly 4 million users and is growing fast, which has Arbel and Hogeg hoping the security flaw can be rectified quickly to allow the simple app to keep moving and allowing users to send out those "yo's" to their friends and contacts.