"Online advertising has become a huge profit center for website owners. Web adverts fuel many free websites, particularly social networks, blogs, forums and news sites. Since Web adverts have a virtually unlimited capacity to reach millions of users, attackers look to them as a doorway to install malicious code.
Advertising space on a website works through scripts that display images and media to lure a user into clicking on them. After taking over the server hosting these advertisements, attackers use URL redirection to send users to a website hosting their malware." – Content courtesy of How to protect against web advertising exploits :: SearchSecurity.com.au