When there are unpatched browser vulnerabilities, attackers will use ad networks to inject attack code into reputable-but-ad-laden websites. And even when there aren't unpatched vulnerabilities out there, many ad networks will happily accept scam ads, ads that trick people into downloading malware, fake download buttons and suchlike.
When there are unpatched browser vulnerabilities, attackers will use ad networks to inject attack code into reputable-but-ad-laden websites. And even when there aren't unpatched vulnerabilities out there, many ad networks will happily accept scam ads, ads that trick people into downloading malware, fake download buttons and suchlike.