Yahoo didn't say how many accounts have been affected.
Yahoo Inc. said in a blog post on its breach that "The information sought in the attack seems to be names and email
That could mean hackers were looking for additional email addresses to send spam or scam messages. By grabbing real names from those sent folders, hackers could try to make bogus messages appear more legitimate to recipients.
Yahoo said the usernames and passwords weren't collected from its own systems, but from a third-party database.
Because so many people use the same passwords across multiple sites, it's possible hackers broke in to some service that lets people 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.
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.