Yahoo Japan said it suspected that up to 22 million of its user IDs may have been "leaked" and it detected an unauthorized attempt to access the administrative system of its web portal Yahoo Japan, the Kyodo news agency reported.
The leaked information does not include passwords and data necessary for identity verification to reset passwords, the agency said.
The company detected the access attempt at around 9pm, Kyodo reported.
Yahoo Japan checked its system after severing access and found traces of an attempt to steal user IDs, according to Kyodo.
Yahoo has a 35 per cent stake in Yahoo Japan.