Opera is alerting customers of its web browser that its sync system was breached.

On a blog post, the Oslo-based company said although it "quickly blocked" an attack on its sync systems, which allows users to share website passwords across devices, it believes "some data, including some of our sync users' passwords and account information, such as login names, may have been compromised."

Although Opera claims that 350-plus million people use its browser, only a small percentage of that base (0.5 percent) are affected by the breach. About 1.7 million active users are said to use Opera's sync account passwords. All passwords are encrypted (for synchronized passwords) or hashed and salted (for authentication) passwords in this system. 

However, the company is urging all users of Opera sync accounts to reset their passwords and is advising all its users, "in an abundance of caution," to reset passwords duplicated for use on third-party sites.