I have spoken about the problems of malvertising before, both here and here.

Ars Techinca has recently reported on a spate of malvertising:
Mainstream websites, including those published by The New York Times, the BBC, MSN, and AOL, are falling victim to a new rash of malicious ads that attempt to surreptitiously install crypto ransomware and other malware on the computers of unsuspecting visitors, security firms warned.