use email addresses as their usernames. The hackers could have grabbed passwords stored at that service, filtered out the accounts with Yahoo addresses and used that information to log in to Yahoo's mail systems, said Johannes Ullrich, dean of research at the SANS Institute, a group devoted to security research and education.
The breach is the second mishap for Yahoo's mail service in two months. In December, the service suffered a multi-day outage that prompted Yahoo CEO Marissa Mayer to issue an apology.
Yahoo said it is resetting passwords on affected accounts and has ''implemented additional measures'' to block further attacks. The company would not comment beyond the information in its blog post. It said it is working with federal law enforcement.