Yahoo Inc. suffered a data breach in 2014 that affected at least 500 million Yahoo user accounts, the company announced Thursday.
The Sunnyvale, Calif., Internet firm said the hacker is believed to be a “state-sponsored actor” and may have gained access to user information such as names, email addresses, telephone numbers, dates of birth, scrambled passwords, and some encrypted and unencrypted security questions and answers.
Yahoo said hackers did not steal unprotected passwords or credit card and bank account information.
The company did not say how long it had known about the data breach. It did not immediately respond to a request for comment.
Yahoo encouraged users to review their online accounts for suspicious activity and to change their passwords and security questions and answers for any other accounts that use the same information. It said it is sending a notice to users it believes may have been affected, and more information is available on its website.
The FBI is aware of the hack at Yahoo and is investigating, according to a U.S. official briefed on the case.
"The FBI has regular contact and good working relationships with our private-sector partners in the Bay Area," said Susan McKee, an FBI spokeswoman. "The compromise of public- and private-sector systems is something we take very seriously, and the FBI will continue to investigate and hold accountable those who pose a threat in cyberspace."
The number of user accounts affected is significant compared with other recent high-profile data breaches. A breach of Target Corp.’s database in 2013 affected more than 100 million customers. EBay Inc. experienced a data breach in 2014 that affected its 148 million customers. Anthem Inc. disclosed in 2015 that it was hacked, and information about more than 70 million customers was compromised.
At the very least, Verizon is going to need more time to assess what it will be getting into if it proceeds with its plans to take over Yahoo, said Scott Vernick, an attorney specializing in data security for the law firm Fox Rothschild.
“This is going to slow things down. There is going to be a lot of blood, sweat and tears shed on this,” Vernick said. “A buyer needs to understand the cybersecurity strengths and weaknesses of its target these days.”
Investors evidently aren't nervous about the Verizon deal unraveling yet. Yahoo's stock rose a penny Thursday to close at $44.17. But the Verizon sale represents a sliver of Yahoo's total market value, which primarily consists of a stake in Chinese e-commerce leader Alibaba Group currently worth $42 billion.
And despite its lagging public image, Yahoo still commands one of the largest online audiences in the world. More than 1 billion people visit its websites every month.
Del Quentin Wilber in Washington and the Associated Press contributed to this report.
3 p.m.: This article was updated to add Yahoo’s stock price and discussion of implications on Yahoo’s deal with Verizon.
12:35 p.m.: This article was updated throughout with Times staff reporting.
This article was originally published at noon.