According to Google Adwords Blog: "On Tuesday, April 24th, Google identified and canceled AdWords accounts displaying ads that re-directed users to malicious sites"
Exploit Prevention Labs has an interesting video that explains the attacks
Exploit’s CTO, Roger Thompson said:
“Google says they are doing the best that they can, but their business model is to take as much money as they can for advertisements. No matter how much due diligence they do, it’s a difficult position to be in, but clearly they are not doing enough,” Thompson said.
“If they don’t do a better job of vetting their customers, we will see this sort of thing happening again and again.”
Looks like Google and other search engines need to overhaul their automated advertising systems!