Foreign hackers, suspected to be agents of a hostile foreign government, have stolen sensitive information from at least 500 million Yahoo accounts, the company announced Thursday.

It was the biggest cyber theft in history — and it took YEARS before the company disclosed the breach.

What should users do?

"First and foremost, you'll want to change your password immediately. All Yahoo account holders should also change their security questions and answers," CNN Money advises. "If your account is one Yahoo suspects was compromised, you'll be prompted to enter a new password as soon as you log on. If you used the same password on other accounts, change those, too.

The startling security breakdown certainly magnifies the tech company's preexisting problems – specifically, that it is losing users, traffic and the advertising revenue that follows both, to rivals such as Google and Facebook.

Some snarky online commentators quipped that the hack would have been far more devastating if people actually still used the company's services. While there's some truth to that observation, millions around the world still rely on Yahoo mail and other services, and are now potentially at risk of identity theft or worse.

And if these people give up on Yahoo as a result, the consequences for the company itself – now scheduled to become part of Verizon as soon as its $4.8 billion deal closes – could also be dire. "Yahoo may very well be facing an existential crisis," said Corey Williams, senior director of products and marketing at the computer security firm Centrify.

Yahoo was already facing a steep decline in email traffic, despite CEO Marissa Mayer's efforts to upgrade the service in order to foster more user loyalty. In July, 161 million people worldwide used Yahoo email on personal computers, a 30 percent decline from the same time in 2014, when the breach first occurred. That's according to the latest data from the research firm comScore. By contrast, Google's rival Gmail service saw desktop users rise 9 percent to nearly 429 million over the same period.

The email breach raises questions about Yahoo's ability to maintain secure and effective services, particularly since it's been laying off staff and trimming expenses to counter a steep drop in revenue over the past eight years.

At the time of the break-in, Yahoo's security team was led by Alex Stamos, a respected industry executive who left last year to take a similar job at Facebook.

Yahoo didn't explain what took so long to uncover a heist that it blamed on a "state-sponsored actor" – parlance for a hacker working on behalf of a foreign government.