Online Ads Could Create ‘Million Browser Botnet’

How a botnet works: 1. A botnet operator sends out viruses or worms, infecting ordinary users’ computers, whose payload is a malicious application — the bot. 2. The bot on the infected PC logs into a particular command and control (C&C) server (often an IRC server, but, in some cases a web server). 3. A spammer purchases access to the botnet from the operator. 4. The spammer sends instructions via the IRC server to the infected PCs, causing them to send out spam messages to mail servers. (Photo credit: Wikipedia)

Want to create a huge botnet to distribute malware, pump out spam, crack passwords or knock your enemy’s website offline?

Don’t bother with designing malware to break into strangers’ computers. Instead, say two researchers, all you need to do is spend a few bucks buying online ads, which can hijack tens of thousands of Web browsers across the world — no hacking required.

Last month at the Black Hat security conference in Las Vegas, Jeremiah Grossman and Matt Johansen, the founder/chief technology officer and threat-research manager of White Hat Security in Santa Clara, Calif., showed how an online ad network could be used to create what they called a “million browser botnet.”

“There’s no malware to detect, no exploits,” Grossman said. “We’re not really hacking stuff. We are using the Web the way it was meant to be used.”

The World Wide Web is a fundamentally insecure system, Grossman and Hansen explained. Browsers are designed to serve you as much data as possible without authentication, and nowhere is that more true than with online ads.

“When you visit a Web page,” Grossman said, “by nature of the way the Web works, it has near-complete control of your browser for as long as you are at that Web page … The JavaScript or Flash on that page can force your browser to do basically whatever it wants.”

Grossman and Johansen showed how HTML and JavaScript, the programming languages underlying most Web pages, could be used to probe Web browsers for user settings and login information, force browsers to attack websites in several different ways, break into corporate networks or spread malware.

The problem with these attacks, however, is that they are limited in scope. Whether you’re distributing the evil code through a highly trafficked site, search-engine poisoning or third-party widgets such as weather trackers, you’re not going to attain the critical mass for a truly efficient browser-based botnet.

“We need to think bigger,” the researchers said, then quoted JavaScript pioneer Douglas Crockford: “The most reliable, cost-effective method to inject evil code is to buy an ad.”

Ads: the perfect malware distribution system

There are nearly two dozen major ad networks, Grossman and Johansen said, but most of them won’t let ad suppliers include code with their ads.  However, there are hundreds of smaller ones that don’t ask as many questions.

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