Yahoo!'s (YHOO) newly reported data breach is more serious than one previously reported. This hack involved 1 billion user accounts and could compromise Yahoo!'s deal to sell its core assets to Verizon (VZ) - Get Report . Yahoo! says the hackers likely stole names, email addresses, telephone numbers, dates of birth and passwords. The latest breach is believed to have occurred in 2013, and is on top of the 500 million hacked accounts from 2014 that were previously disclosed. News of the breach weighed on Yahoo!'s stock.