The bad news is if you click on the wrong online ad, your browser can be immediately enlisted in a botnet carrying out a denial of service attack to take down Web sites.
The good news is that as soon as you move on to another Web site, the browser is released with no harm done, according to researchers who revealed the hack at the Black Hat security conference.
"Who's problem is this?" says Jeremiah Grossman, CEO of White Hat Labs and one of the researchers. "Browsers? Ad networks? Who fixes this?"
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"To scale [the botnet] up you need to get a lot of browsers running it," he says.
The researchers paid the ad network to distribute their ad and within 18 hours it was generating 8.1 million requests to the server coming in fast enough to take it down. That was using HTTP requests six at a time without using the FTP bypass, Grossman says. Since the users whose browsers were enlisted to the botnet were unwitting, they didn't want to make any changes to the browsers, he says.
The upside for attackers is that the botnet is random with no command-and-control server that defenders could take down. Grossman says he is uncertain whether it would be possible forensically to track down the ad at the center of such a botnet and ultimately track it to the individuals who bought the ad. "You could be tracked by who paid for the guilty ad," he says.
Ad blockers that are used to speed up the loading of Web pages and make them less annoying to users could become a security tool if this technique catches on, Grossman says, but he didn't have a way to stop such attacks. "We used the way the Web works and took down our own server," he says.
Tim Greene covers Microsoft and unified communications for Network World and writes the Mostly Microsoft blog. Reach him at firstname.lastname@example.org and follow him on Twitter @Tim_Greene.
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This story, "Black Hat: How to Create a Massive DDoS Botnet Using Cheap Online Ads" was originally published by Network World.