UPDATE: Yo has now responded to the claims - see below.
'Yo' has barely been a viral sensation for more than 24 hours, but it might have already been hacked.
Yo is a deliberately, mind-numbingly simple communications app, which allows you to send the same two-character message to anyone with the app installed.
It's really not any more complex than that.
Nevertheless, users are reporting issues with the app which range as far as having various meme-tastic tunes play instead of the usual notification sound.
A Vine -- unverified at this stage -- has been posted showing the app sending out Rick Astley's 'Never Gonna Give You Up' instead of the normal sound:
Meanwhile Techcrunch reports a student has been able to break into the app itself and spoof messages.
"We can get any Yo user’s phone number (I actually texted the founder, and he called me back). We can spoof Yo’s from any users, and we can spam any user with as many Yo. We could also send any Yo user a push notification with any text we want (though we decided not to do that)."
Yo has now responded to the claims with the following Tweet:
We working on the securities issues that came to our attention. We want you to know we take this very seriously.— Yo (@YoAppStatus) June 20, 2014