Creating Browser-Based Botnets Through Online Ad Networks

LAS VEGAS -- BLACK HAT USA -- For several years security researchers and black hat hackers have fine-tuned methods of manipulating the eccentricities and vulnerabilities of the way browsers work to make user machines visit certain sites, download illegal content, and even carry out attacks like SQL injection without the user knowing it. However, these attacks have always been thought of as invoking one-off behavior that wouldn't scale well enough to leverage for something like a distributed denial-of-service attack (DDoS). But yesterday at Black Hat USA, a pair of researchers showed it is possible to maneuver browsers on a massive scale through online advertising.

In a demo at their session, WhiteHat Security researchers Jeremiah Grossman, CTO and founder, and Matt Johansen, manager of the firm's threat research center, showed it's possible to essentially create a hard-to-trace browser botnet that can easily trigger DDoS with a minimal investment in a fake online ad. As they explained, networks that serve up advertisements on ad-supported sites across the Internet frequently allow their advertisers to run arbitrary JavaScript on browsers displaying their ads. Using JavaScript to make hundreds of thousands or millions of advertising viewers connect at once to a particular target site could quickly create enough connections to take down most sites on the Web.

"The Web runs on advertising -- that's how all these websites are paid for," Grossman said. "So the reach of these advertising networks is phenomenal."

In their demonstration, Grossman and Johansen purchased $20 worth of advertising impressions through an unnamed advertising network and placed an innocuous-looking ad with a call to automatically load an external site where they could change their JavaScript payload on the fly.

"The hardest part of all this research was the approval process. Not for the reasons you'd think -- they happened to not be very good at reading JavaScript, or even caring about JavaScript. What they actually cared about was that the ad looked pretty and worked like an ad," Johansen said. "But anytime we wanted to tweak something, like change a URL, it had to go through reapproval. So instead of putting the code directly in the ad, we just put script source and sourced it out to a file on our side."

The researchers stood up an Apache server on AWS to crash it in front of the audience within a few seconds of targeting their script-running browsers toward it.

"This whole time we did not hack anybody. We just used the way the Web works and took down our own service," Johansen said. "We stayed completely on the legal side here. But you can kind of get an idea of how this could get fun if you didn't."

Not only could malicious hackers do much more damage with more malicious code, but "there's no particular reason why the bad guys couldn't use a stolen credit card" to carry out this kind of attack, Grossman said.

According to Johansen, the advantage to an attacker of using this method is its disappearing footprint.

"So why not just do a traditional denial-of-service attack? It's not persistent. It goes away," he says. "There's no trace of this -- we put the money in the machine, the JavaScript gets served up, and then it goes away. And it's very, very easy." Have a comment on this story? Please click "Add Your Comment" below. If you'd like to contact Dark Reading's editors directly, send us a message.