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Your crazy next-door neighbor could also be your biggest WiFi security risk: If his wireless router gets infected, it could spread malware to your router as well.

Researchers at Indiana University in Bloomington recently built an epidemiological model of the feasibility of malware rapidly spreading among WiFi routers in a densely populated area. They found in their simulations that tens of thousands of WiFi routers could be infected in two weeks -- and most were infected within 24 to 48 hours.

The researchers then tested their model as a launching pad for drive-by pharming, an attack method demonstrated last year by several of their colleagues in a proof-of-concept attack they co-developed with Symantec. That's where an attacker uses a broadband router vendor's default password to control the router, after first luring the victim to a malicious Website to infect them with malicious JavaScript code. (See New 'Drive-By' Attack Is Remote.)

"I [wondered], why would you be content with that when you could make it self-replicating and attack neighboring routers?" says Steven Myers, assistant professor of Indiana University's School of Informatics, and one of the members of the research team.

So Myers, along with university colleagues Hao Hu, Vittoria Colizza, and Alessandro Vespignani, gathered wireless data from seven urban areas in the U.S. -- Chicago, Boston, New York City, San Francisco Bay Area, Seattle, Northern Indiana, and Southern Indiana – and modeled a malware viral epidemic among WiFi routers akin to the spread of a biological virus.

They found that only 21 percent to 40 percent of wireless routers in those regions used encryption, and they focused on "attacking" those without encryption or with the notoriously weak Wireless Encryption Protocol (WEP). The researchers concluded that WEP can always be broken as long as the attacker has access to sufficient encrypted data.

"This can be achieved by waiting for the router to be used by legitimate clients, or by deploying more advanced active attacks," they wrote. "Bypassing WEP encryption is therefore feasible and only requires a given amount of time."

WEP's weaknesses are well documented. McAfee researchers found that half of the home WLANs they sniffed in war-driving tests over a year ago were unprotected and unencrypted. The team was able to crack those that had WEP encryption on their WiFi routers in less than two minutes. (See Hacking Home WLANs.)

Once WEP is cracked, the attacker is practically "in": Many WiFi routers still use their manufacturer's default password. The researchers weren't able to nail down the number of those users, but used estimates which state that 25 percent of users don't change their router's default SSID. An infected router would then interface with another router over an administrative interface, according to the researchers, who tested that out as well.

Myers says the danger of this type of attack is that, unlike a PC, a wireless LAN is typically "on" all the time, and no security tools today specifically protect WiFi router infection. Still, it's not an active threat at this time: The goal of the researchers is to help get ahead of this type of threat before it becomes a problem.

"I don't think it's a threat right now -- there's so much more lower-hanging fruit" for attackers to exploit, he says. But this attack would be attractive for script injection, pharming, and for botnet operators to build out their botnets, he says.

"This is a whole other platform to infect and attack," Myers says.

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