Malware in Google ads

From Websense warning that some links on Google ads will lead to malicious sites where you get to install malware on your computer. So some links that offer Winrar, a tool for file compression, users will lead to pages that get to download and install malicious software that changes the page that the user opens so instead of Google, Yahoo and other sites open pages that control the attackers on a computer . Such links are a great danger for users because Hacking & Cracking, instead of exploiting the security failure, now deliberately malware through legitimate ads on Google, where would never thought that the word about the fraud. This situation leads to the question - who is to blame for this situation? Google, which does not test the credibility of links or users themselves, who eventually drop Fraud Bin victims. In any case, the Google want to avoid such problems in the future and are working to remove links that carry malware from its advertising network.