Yahoo says hackers stole data on 500 million users in 2014.

• Position: CYBERSECURITY

Experts have called it the biggest data breach to date. At least 500 million Yahoo users' account information was stolen by hackers in 2014.

In a statement, Yahoo said user information--including names, email addresses, telephone numbers, birth dates, encrypted passwords, and, in some cases, security questions--was compromised in 2014 by what it believes was a "state-sponsored actor."

According to the New York Times, Yahoo is one of the Internet's busiest sites, with one billion monthly users and one of the oldest free e-mail services. Many users have built their digital identities around it, from bank accounts to photo albums and even medical data.

"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.

"This is one of the biggest breaches of people's privacy and very far-reaching."

Upon discovering the breach--two years after it occurred--Yahoo instructed users to change their passwords and stay vigilant over their other online accounts. Yahoo said it was working with law enforcement agencies in their investigations.

Yahoo said it learned of the data breach this summer after hackers posted to underground forums and online marketplaces what they claimed was stolen Ya hoo...