Editors' pick: Originally published Sept. 22.

Yahoo! (YHOO) is grappling with one of the largest data breaches in history, with over 500 million user accounts stolen in 2014. The company, headed by Marissa Mayer confirmed Thursday, Sept. 22, that "certain user account information was stolen" by "what it believes is a state-sponsored actor."

The hack may imperil Yahoo!'s pending acquisition by Verizon (VZ) - Get Report for $4.8 billion. The deal, announced in July after the Internet company spent several months on the auction block, was predicated on Yahoo!'s active user base of over 1 billion.

"Online intrusions and thefts by state-sponsored actors have become increasingly common across the technology industry," Yahoo! said in a statement. "Yahoo! and other companies have launched programs to detect and notify users when a company strongly suspects that a state-sponsored actor has targeted an account."

After you're done frantically changing your old Yahoo! account password, take a look at some of the largest data breaches in recent years.