In a message on its website, the company explains that earlier this week, the sync service was under attack. Opera blocked the attack but warns that some user data, including password and account information may have been compromised:
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 sync has an active userbase of around 1.7 million users.
Although we only store encrypted (for synchronized passwords) or hashed and salted (for authentication) passwords in this system, we have reset all the Opera sync account passwords as a precaution.
We have 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 synchronized with the service.
To obtain a new password for Opera sync, use the password resetting page.