Yahoo has said it has identified a new system breach that occurred in August 2013 and involved data associated with more than one billion user accounts.
The company said it believed this incident is likely distinct from the breach it disclosed in September, when information associated with at least 500 million user accounts was stolen from its network in 2014.
Yahoo, which is being acquired by Verizon, said an unauthorized third party had stolen the data in the latest breach and that it was working closely with law enforcement.
Yahoo said the stolen user account information may have included names, e-mail addresses, telephone numbers, dates of birth, hashed passwords and, in some cases, encrypted or unencrypted security questions and answers.
Payment card data and bank account information were not stored in the system believed to be affected, the company said.