Which 2013 breach exposed the personal data of about 3 billion Yahoo accounts?

The 2013 breach that exposed the personal data of about 3 billion Yahoo accounts was the Yahoo data breach.

Yahoo announced in 2016 that attackers had stolen information associated with at least 500 million accounts in a 2014 intrusion. In 2017, the company revised its assessment of a separate 2013 incident, saying that information from all of its approximately 3 billion accounts had been affected. The stolen information included account details such as names, email addresses, telephone numbers, dates of birth, and security questions, while unprotected passwords and payment-card data were not reported as part of that particular theft.

The incident is often discussed as two major Yahoo breaches rather than one event. That distinction explains why news reports cite both 500 million and 3 billion. The breaches also contributed to a reduction in Verizon's purchase price for Yahoo.

The case illustrates why account recovery questions and reused passwords can remain dangerous even after a breach becomes public.