Hackers have posted what they say are usernames and partial phone numbers of 4-point-6 million users of mobile photo-sharing service Snapchat.
A group says it published the data on a website in an effort to convince makers of the app to improve its security.
Snapchat lets users send photos and videos, which disappear shortly after they're viewed.
Conor Flynn an information security expert - he's been explaining the background to the story:Meanwhile, Skype has apologised to its users after its Twitter and Facebook pages were hacked.
Messages posted to the pages claimed that Microsoft was monitoring its users' email accounts and selling the data to governments.
The 'Syrian Electronic Army', a group of hackers who support president Bashar Assad, has claimed responsibility.
It's published the phone number of Microsoft CEO Steve Ballmer, encouraging users to call him and complain.
