A breach at Yahoo Japan, the most visited website in the country, may have exposed up to 22 million user login names.
Yahoo said in a news release that on Thursday it discovered an unauthorized access to its ID servers, as well as a file hosting the login names.
The company said it “can't deny the possibility” that the file was transferred.
Although login names are public information, as an added precaution, the company has urged users to change their passwords, as well as use an added tool on its home page that allows them to check if their login ID has been placed at risk as a result of the incident.
In 2012, Yahoo made headlines after hackers stole about 400,000 usernames and passwords of members who belong to the company's Contributor Network.