Yahoo Says 1 Billion User Accounts Were Hacked

SAN FRANCISCO — Yahoo, already reeling from its September disclosure that 500 million user accounts had been hacked in 2014, disclosed Wednesday that a different attack in 2013 compromised more than 1 billion accounts.

The two attacks are the largest known security breaches of one company’s computer network.

The newly disclosed 2013 attack involved sensitive user information, including names, telephone numbers, dates of birth, encrypted passwords and unencrypted security questions that could be used to reset a password. Yahoo said it is forcing all of the affected users to change their passwords and it is invalidating unencrypted security questions — steps that it declined to take in September.

It is unclear how many Yahoo users were affected by both attacks. The internet company has more than 1 billion active users, but it is not clear how many inactive accounts were hacked.

Yahoo said it discovered the larger hacking after analyzing data files, provided by law enforcement, that an unnamed third party had claimed contained Yahoo information.

Security has taken a back seat at Yahoo in recent years, compared to Silicon Valley competitors like Google and Facebook. Yahoo’s security team clashed with top executives, including the chief executive, Marissa Mayer, over the cost and customer inconvenience of proposed security measures.