Yahoo announced Wednesday that more than one billion user accounts may have been affected in a hacking attack dating back to August 2013, a revalation that may put a recent multi-billion-dollar deal with Verizon in jeopardy.
The Sunnyvale, California, says it believes information, including names, email addresses, phone numbers, birth dates and security questions and answers, may have been stolen in the breach. The company says it believes bank-account information and payment-card data were not affected.
The security breach was discovered after law enforcement officials provided the company with data files that a "third party" claimed was Yahoo user data. After analyzing the information, Yahoo concluded that the data belonged to its users.
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Yahoo says it has not been able to identify the source of the hack.
"Yahoo is notifying potentially affected users and has taken steps to secure their accounts, including requiring users to change their passwords. Yahoo has also invalidated unencrypted security questions and answers so that they cannot be used to access an account," the company said in a statement.
Yahoo is also urging users to avoid clicking links or downloading attachments from suspicious emails, and to be cautious of unsolicited communications that ask for personal information.
The tech giant says it's a different breach from the one it disclosed in September, when it reported that 500 million accounts were exposed.
Based on the ongoing investigation, the company believes hackers accessed the company's proprietary code to learn how to forge cookies that could allow an intruder to access users' accounts without a password. The company says it has connected some of this activity to the same "state-sponsored actor" believed to be responsible for the data theft the company disclosed on September 22, 2016.
The new hack revelation raises questions about whether Verizon will try to change the terms of its $4.8 billion proposed acquisition of Yahoo.
If the hacks cause a user backlash against Yahoo, the company's services wouldn't be as valuable to Verizon, raising the possibility that the sale price might be re-negotiated or the deal may be called off. The telecom giant wants Yahoo and its many users to help it build a digital ad business.
After the news of the first hack broke, Verizon said it would re-evaluate its Yahoo deal and in a Wednesday statement said it will review the "new development before reaching any final conclusions." Spokesman Bob Varettoni declined to answer further questions.
The Associated Press contributed to this report.