Experts at InfoArmor said the stolen database of 1 billion Yahoo accounts has been sold multiple times for at least $300,000 each time.

Yahoo has announced it will follow in the footsteps of Twitter and Facebook and begin warning users when it believes their accounts have been targeted by a state-sponsored actor. Bob Lord, who was hired as the company’s new CISO in October, discussed the initiative in a blog post Monday. Lord said Yahoo will only notify users...