A data breach in August 2013 exposed information held in more than 1 billion Yahoo user accounts, the company said Wednesday.
The hacking incident is separate from an intrusion Yahoo reported in September, when the company disclosed 500 million user accounts were exposed in 2014.
"Yahoo believes an unauthorized third party, in August 2013, stole data associated with more than one billion user accounts," Yahoo said in a statement. "Yahoo believes this incident is likely distinct from the incident the company disclosed on September 22, 2016."
The information stolen may include names, email addresses, phone numbers, birthdates and security questions and answers. Payment card and bank account data were not believed to have been affected, the company said.
Yahoo, based in Sunnyvale, California, is in the process of being acquired by Verizon in a $4.8 billion deal. The company did not identify the "unauthorized third party" involved in the 2013 hacking.