Tarquin Wilton-Jones writes on Opera’s blog:
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.
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.
Read more on Opera.