Opera's Sync service, which allows users synchronise their browsing data across different devices, has a total active userbase of about 1.7 million.
"Earlier this week, we detected signs of an attack where access was gained to the Opera sync system. This attack was quickly blocked. Our investigations are ongoing, but we believe some data, including some of our sync users' passwords and account information, such as login names, may have been compromised," Opera said in a blogpost.
Opera said it has also sent emails to all Opera sync users to inform them about the incident and ask them to change the password for their Opera sync accounts.
"In an abundance of caution, we have encouraged users to also reset any passwords to third party sites they may have synchronised with the service," it said.
The total active number of users of Opera sync in the last month was 1.7 million, less than 0.5 per cent of the total Opera user base of 350 million people.