Criminals could target unsecured wireless routers to create an attack that could piggyback across thousands of Wi-Fi networks in urban areas such as London or New York City, according to researchers at Indiana University in the US and the Institute for Scientific Interchange in Torino, Italy.

A Wi-Fi attack could take over 20,000 wireless routers in New York City within a two-week period, with most of the infections occurring within the first day, according to the researchers. Visit Security Advisor for the latest PC security news, reviews, tips and tricks.

The researchers theorise that the Wi-Fi virus attack would work by guessing administrative passwords and then instructing the routers to install new worm-like firmware which would in turn cause the infected router to attack other devices in its range.

"The issue is that most of these routers are installed out of the box very insecurely," said Steven Myers, an assistant professor at Indiana University, who published the paper in November.

Because there are so many closely connected Wi-Fi networks in most urban areas, the attack could hop from router to router for many miles in some cities.

The team used what is known as the Susceptible Infected Removed (SIR) model to track the growth of this attack. This methodology is typically used to estimate things such as influenza outbreaks, but it has also been used to predict computer virus infections, Myers said.

Although the researchers did not develop any attack code that would be used to carry out this infection, they believe it would be possible to write code that guessed default passwords by first entering the default administrative passwords that shipped with the router, and then by trying a list of one million commonly used passwords, one after the other. They believe that 36 percent of passwords can be guessed using this technique.

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Even some routers that use encryption could be cracked, if they use the popular WEP (Wired Equivalent Privacy) algorithm, which security experts have been able to crack for years now. Routers that were encrypted using the more-secure WPA (Wi-Fi Protected Access) standard were considered impossible to infect, Myers said.

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