Yahoo Japan, the country's largest Web portal, said up to 22 million user IDs may have been leaked during a hack that was discovered last week.
The company emphasized that the IDs are already public information, and no passwords or other private data were affected. Yahoo Japan IDs are used along with password to log in to the site, and are often displayed when users leave comments or use its shopping or auction services.
Yahoo Japan said it discovered illicit access to its ID servers on Thursday evening, and upon further investigation found a file with 22 million user IDs on it. The company said it wasn't sure if the file had been transferred outside of the company, but couldn't deny the possibility.
The website posted warnings of the possible breach on its login pages, and offered a service for users to check if their IDs were among those that were possibly leaked. Yahoo Japan said last year it had over 24 million active user IDs.
Yahoo Japan does not allow users to change their IDs without creating an entirely new account, which means losing access to existing mail and other data. The company does allow creation of a secondary user ID it calls a "Secret ID," which is used solely for logging in and not meant to be shared publicly.
The company introduced the Secret ID feature as part of a security upgrade after a security breach last month. Yahoo Japan said it had discovered a malicious program on company servers that had extracted user data for 1.27 million users, but the program was stopped before it leaked any of the data outside of the company.
Yahoo Japan is the country's most-visited Web property, according to Web data provider Alexa, and the 15th most visited site globally. It is majority owned by Softbank, which also runs one of the country's largest mobile phone operators and a large broadband service. Yahoo holds a 35 percent stake in the portal.