Online advertising has become an increasingly potent threat to end-user security on the internet. More hackers than ever are targeting the internet's money engine, using it as a powerful attack vector to hide exploits and compromise huge numbers of victims.
Malvertising, as poisoned ads are known, is as deadly as it is diverse. Hackers are able to poison advertisements with the world's most capable exploit kits, then pay to have it served on a large number of prominent websites. Up to half of users exposed to the very worst forms of malvertising fall victim, yet tracking the attacks is often tricky. Advertisements are dynamic and served only to certain users, on certain websites, in certain conditions, making attacks difficult to study.
http://www.theregister.co.uk/2015/08/27/malvertising_feature/