Drive-By Downloads: Malware's Most Popular Distribution Method

WASHINGTON, D.C. -- OWASP AppSec DC 2010 -- Why try to fool users into opening email attachments when you can simply drop a Trojan on them from their favorite websites?

That's the question many malware authors and distributors are asking -- and the obvious answer is spurring most of them to try out the emerging "drive-by download" method, according to a speaker here this week.

"What we're seeing is a fundamental change in the method of malware distribution," said Neil Daswani, CTO of Dasient, which offers a service that detects and eradicates Web-borne malware. "In the old days, we saw executable code in a static file, which was originally delivered via floppy disks and then via email attachments. Now we're seeing active content delivered via drive-by downloads at legitimate sites."

A drive-by download typically begins by injecting a Web page with malicious code, often through JavaScript, Daswani explained. The code generally invokes a client-side vulnerability to deliver shell code, such as the JavaScript-based Heap Spray attack, to take control of the user's machine. From there, the attacker can send a "downloader," which is often custom, zero-day code that isn't recognized by traditional antivirus systems.

Once the downloader is in place, the attacker can deliver his malware of choice, Daswani said. Drive-by downloads are particularly effective for delivering code that can steal end user credentials (such as Zeus), launch a fake antivirus scam (such as Koobface), steal server-side administrative credentials (such as Gumblar), steal corporate secrets (such as Project Aurora), or collect fraudulent click revenue (such as clickbot.A), he noted.

While drive-by downloads are often more effective at infecting end user devices than email attachments, they also give the attacker broader reach, Daswani observed. Drive-by downloads can be used to infect thousands of websites at once, often by hiding in common third-party devices that are distributed to many sites, such as advertisements, widgets, images, or third-party applications.

"A lot of user organizations do a great job of scanning the code they put on their own sites, but they may not scan the code they're posting from third parties," Daswani warned. "The marketing people will add an ad or a widget to a site, and the IT people may not vet it before it's posted."

Many well-known sites are infected by malware, and the most popular sites are generally targeted most frequently, Daswani noted. In the past two years, major government sites, such as the Treasury Department and Environmental Protection Agency, have been infected, causing them to serve up drive-by downloads to their users. The National Institute of Health has been infected five times in the past two years, and the state of Alabama's website has been infected 37 times in that same time period, he reported.

"It's time to recognize that this is the method of choice for many distributors of malware," Daswani said.

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