Snapchat hack affects 4.6 million users
2 January 2014 Last updated at 21:03 GMT
The usernames and phone numbers for 4.6 million Snapchat accounts have been downloaded by hackers, who temporarily posted the data online.
The hack comes days after a security firm publicly warned of weaknesses in Snapchat's app, which it said could be exploited by hackers.
Rory Cellan Jones reports.