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," Yahoo revealed in a statement late Thursday.
"The ongoing investigation suggests that stolen information did not include unprotected passwords, payment card data, or bank account information; payment card data and bank account information are not stored in the system that the investigation has found to be affected," the Sunnyvale, California-based company added.
According to Yahoo's Chief Information Security Officer Bob Lord, the information was stolen from the company's network in late 2014.
Based on the ongoing investigation, Yahoo believes that information associated with at least 500 million user accounts was stolen and the investigation has found no evidence that the state-sponsored actor is currently in Yahoo's network.
Yahoo is working closely with law enforcement on this matter. The breach could also impact Yahoo's $4.8 billion sale of its core business to Verizon.
Yahoo has notified potentially affected users and has taken steps to secure their accounts.
These steps include invalidating unencrypted security questions and answers so that they cannot be used to access an account and asking potentially affected users to change their passwords.
Yahoo has recommended that users who have not changed their passwords since 2014 to do so.
"Yahoo encourages 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," the statement read.
The company further recommends that users avoid clicking on links or downloading attachments from suspicious emails and that they be cautious of unsolicited communications that ask for personal information.
Additionally, Yahoo has asked users to consider using Yahoo Account Key, a simple authentication tool that eliminates the need to use a password altogether.
Online intrusions and thefts by state-sponsored actors have become increasingly common across the technology industry including professional social network LinkedIn, micro-blogging website Twitter and file hosting service Dropbox, to name a few.
Yahoo and other companies have launched programmes to detect and notify users when a company strongly suspects that a state-sponsored actor has targeted an account.
Since the inception of Yahoo's programme in December 2015, independent of the recent investigation, approximately 10,000 users have received such a notice.
