Hundreds of accounts connected to smartphone messenger application, Line was reported to have been hacked.
The account was accessed by anonymous hackers. LINE has urged users to change their passwords.
A Line spokesman told AFP that at least 303 cases of unauthorised access were confirmed between late May and June including three that involved cash trades resulting in financial loss.
The accounts were hacked “presumably after shared passwords with other online services were leaked somewhere else,” he said.
Famous for its “stickers” which users can post to their friends, Line now has more than 400 million users, mainly in Japan and mainland Asia, and is growing fast.
The service allows users make free calls, send instant messages and post funny photos or short videos, combining attributes from Facebook, Skype and messaging application WhatsApp.
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