Yahoo (2013-2014)

All 3 billion Yahoo accounts were compromised in the largest data breach ever — a staggering 6x more than Marriott International’s 500 million-record leak. Attackers stole names, email addresses, phone numbers, and hashed passwords across a 2013–2014 campaign that Yahoo disclosed only in 2016. That total represents 93% of the world’s internet users at the time, dwarfing every competitor. The aftermath included a $350 million reduction in Verizon’s acquisition price and years of regulatory scrutiny.