The login info of around 22 million Yahoo Japan users may have been stolen during a hacking attack on May 16.
“We do not know if the file was leaked or not, but we cannot deny the possibility given the volume of traffic between our server and external” terminals, the company said on Friday.
The company detected the attack based on the volume of traffic between back end administration systems and the external internet. Around 22 million login names were hacked, more than a tenth of Yahoo Japan’s 200 million members, the international press reports.
In order to prevent such incident to repeat, the company advised users to reset their passwords, despite passwords or identity verification data not being taken, and said it has “updated its security measures”.
It is not clear yet, according to the company’s officials, how long the attack took or how it happened.
Yahoo Japan, a collaboration between Japan’s SoftBank and the US Yahoo!, is currently the top search engine in the country with a 50 per cent market share; Google holds around 40 per cent share.
The company is 35.5 percent held by Japan’s mobile phone operator SoftBank, and 34.7 percent held by U.S. Internet giant Yahoo! Inc.