The “Malvertising” hack used Yahoo server advertisements to infect computers. The Hack used an unpatched flash exploit to infect Yahoo users computers.
According to a Kapersky Labs, Malvertising is a
“malicious variety of online advertisements generally used to spread malware … While it’s easy to call an ad that redirects to malware a malicious one, it is often hard to differentiate between fraudulent and legitimate online ads..”
According to the security firm who reported the hack, it’s one of the largest malvertising hacks in history based on Yahoo’s 6.9 billion monthly users. This attack used Yahoo ad’s and an unpatched flash exploit against Yahoo’s users and incases installed “Ransomware” which demanded payment before unlocking the infected computer.
Yahoo has since patched it’s ad software.