Security problems involving banner advertising networks go beyond the recent distributed denial of service (DDoS) attack on DoubleClick. Banner networks, with their ability to place code on hundreds of outside sites, offer a vehicle for the rapid distribution of trojans and other malware, as well as a way to deface web pages. In a troubling development, phishing scams have recently demonstrated the ability to install keylogging trojans via banner ads.
The attack on DoubleClick caused performance problems for the network’s clients. But in recent weeks, several smaller banner networks have been used to inject malicious code into web sites. In each case, the banner code serves as the trigger for a string of exploit scripts that trick Internet Explorer into downloading malware or spyware.