Yahoo! Inc. has confirmed a massive data breach that is impacting 500 million accounts, the California-based company announced Thursday.

A recent investigation by Yahoo confirmed that a copy of certain user account information was stolen from the company's network in late 2014 by what it believes is a state-sponsored actor, the company said in a news release.

"The account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords (the vast majority with bcrypt) and, in some cases, encrypted or unencrypted security questions and answers," a news release states.

The investigation, Yahoo said, suggests stolen information didn't include unprotected passwords, payment card data or bank account information. Payment card data and bank information are not stored in the system that the investigation has found to be impacted, the company said.

Yahoo believes information associated with at least 500 million user accounts was stolen. The investigation has found no evidence that the state-sponsored actor is currently in Yahoo's network.

Yahoo is encouraging users to review their online accounts for suspicious activity and to change their password and security questions and answers for any other accounts on which they use the same or similar information used for their Yahoo account.

Source: Graphiq