According to USA Today,
Yahoo revealed Thursday (September 22nd) that it had been hacked in 2014 by what it believes was a state-sponsored actor, with information stolen from at least 500 million Yahoo accounts. Information that may have been taken includes names, email addresses, phone numbers, birth dates, and in some cases, encrypted or unencrypted security questions and answers. The size of the data breach is believed to be the largest ever. Yahoo said it was notifying potentially affected users and taking steps to secure their accounts, and stated that users who haven’t changed their passwords since 2014 should do so.
Wow, that’s scary.