Yahoo Japan suspects up to 22 million user IDs may be stolen.
The company had detected an unauthorized attempt to access the administrative system of its Web portal Yahoo Japan at around 9pm Japan time Thursday, Reuters reported on Friday, citing Kyodo News Agency.
The leaked information does not include passwords and data necessary for identity verification to reset passwords, according to Kyodo News.
The company has checked its system after severing access and found traces of an attempt to steal user IDs, the newswire said.
Yahoo Japan is 35.5 percent held by Japan's mobile phone operator SoftBank, and 34.7 percent held by U.S. Internet giant Yahoo.
Earlier in April, a hacker had also gained unauthorized access to a system managing the Yahoo Japan portal website and came close to stealing information such as IDs, e-mail addresses and passwords on at least 1.27 million accounts, a separate report by Kyodo News noted.