Google's DoubleClick ad network has once again been caught distributing malicious banner displays, this time on the home page of eWeek.
Unsuspecting end users who browse the Ziff Davis Enterprise Holdings-owned site were presented with malvertisements with invisible iframes that redirect them to attack websites, according to researchers at Websense. The redirects use one of two methods to infect users with malware, including rogue anti-virus software.
Once users were redirected, the site dropped a series of malicious files, including one named winratit.exe, into a user's temporary files folder and then prompted them to be automatically called the next time the machine rebooted. The result was the installation of Anti-Virus-1. It invites users to divulge their payment details and also alters their host file to make it hard to disinfect the machine.
The scourge of malvertisements has been a lingering threat over the past few years, and as the world's biggest ad network, DoubleClick has repeatedly been caught playing an unwitting role. Booby-trapped banner ads are the perfect vector because they hit users while visiting trusted sites, so their guard is down.
Catching the tainted banners has been challenging for DoubleClick and its competitors because the perpetrators often go to great lengths to conceal their activities. Miscreants often set up fictitious advertising agencies that appear to be legitimate. They also have the ability to turn the attacks on and off at the drop of a dime to evade sensors seeking out the malicious ads.
Given DoubleClick's tremendous reach, it's possible the rogue ads have shown up on websites other than eWeek, Hubbard said.
A Google spokesman said: "Our scanners have found a few instances of these malware ads in the DoubleClick network. As such, we've added these domains to our malware list and are in the process of removing any offending ads from our ad network."
He declined to say how long the attacks had been active, how many websites they affected or how the attackers were able to bypass Doubleclick's defenses. ®