SAN FRANCISCO — Yahoo announced Thursday that the account information for at least 500 million users was stolen by hackers two years ago, in the biggest known intrusion of one company's computer network.
In a statement, Yahoo said user information — including names, email addresses, telephone numbers, birth dates, passwords and, in some cases, security questions — was compromised in 2014 by what it believed was a "state-sponsored actor." It did not name the country involved.
The company said that it is working with law enforcement officials and that it is invalidating existing security questions and asking users to change their passwords. Yahoo also encouraged people to review other online accounts for suspicious activity, change passwords and security questions on those accounts, and watch out for suspicious emails.
Verizon Communications is moving forward with a $4.8 billion acquisition of Yahoo, which was announced in July. It is unclear what effect, if any, the breach will have on Yahoo's sale price.
Yahoo said it learned of the data breach this summer after hackers posted to underground forums and online marketplaces what they claimed was stolen Yahoo data. A Yahoo team investigated the data and was unable to confirm that the stolen data had originated from a breach at Yahoo. But in investigating the security of its systems, the company discovered that there was another breach, by what it believes was a state-sponsored actor, that dated back to 2014.
Security experts say the breach could have major consequences.
"The stolen Yahoo data is critical because it not only leads to a single system but to users' connections to their banks, social media profiles, other financial services and users' friends and family," said Alex Holden, the founder of Hold Security, which has been tracking the flow of stolen Yahoo credentials on the underground Web.
Two years is an unusually long time to identify a hacking incident. According to the Ponemon Institute, which tracks data breaches, the average time it takes organizations to identify such an attack is 191 days, and the average time to contain a breach is 58 days after discovery. Security experts say the breach could bring about class-action lawsuits, in addition to other costs.