"Web ads are becoming a delivery system of choice for hackers seeking to distribute viruses over the Internet.

In a development that could threaten the explosive growth of online advertising, hackers have started to exploit security holes in the online-advertising chain to slip viruses into ads. Just going to a site that shows such an ad can infect a user's computer.

In May, a virus in a banner ad on tomshardware.com automatically switched visitors to a Web site that downloaded "malware" -- malicious software designed to attack a computer -- onto the visitor's computer. ScanSafe Inc., one of the first security firms to discover the virus, estimates the banner ad was on the site for at least 24 hours and infected 50,000 to 100,000 computers before Tom's Hardware removed it."

Article Link: http://online.wsj.com/public/article/SB1184806085008...