Yahoo confirmed on Thursday data "associated with at least 500 million user accounts" have been stolen in what may be one of the largest cybersecurity breaches ever, CNN informs. 

The company said it believes a "state-sponsored actor" was behind the data breach, meaning an individual acting on behalf of a government. The breach is said to have occurred in late 2014.

"The account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords (the vast majority with bcrypt) and, in some cases, encrypted or unencrypted security questions and answers," Yahoo said in a statement.

Yahoo urges users to change their password and security questions and to review their accounts for suspicious activity.

Yahoo is working with law enforcement to learn more about the breach.

"The FBI is aware of the intrusion and investigating the matter," an FBI spokesperson said. "We take these types of breaches very seriously and will determine how this occurred and who is responsible. We will continue to work with the private sector and share information so they can safeguard their systems against the actions of persistent cyber criminals."

Yahoo originally said it was "aware of a claim" and was investigating the situation. Nearly two months later, it turns out the situation is even worse.

The mega-breach could create a headache for both companies, including damaging press, scrutiny from regulators and a user exodus, just as they're working to close the deal and figure out the future of Yahoo.