Researchers at Indiana University have warned of the increased opportunity for hackers resulting from the spread of Wi-Fi. The researchers were specifically talking about the new breed of city-wide Wi-Fi networks where thousands can be logged on simultaneously, but the principle can also be applied to smaller-scale public hotspots.

What's more, the researchers think such a threat could piggyback across multiple Wi-Fi networks too, taking over thousands of networks in one fell swoop. The researchers think such an attack would easily spread by guessing admin passwords that simply haven't been changed. In fact, they estimate that 36 per cent of passwords could be guessed.

"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 told the IDG News Service. He's got a point; most manufacturer's instructions don't even tell you how to change the admin username and password from the default settings, meaning anybody who can access your wireless network can change your settings. And if you don't have security enabled, that's bad news. Even if you have, it might not stop such an attack: WEP encryption is old news to hackers.

The potential attack could install new firmware on routers, meaning they could in turn attack other networks. However, there is one caveat to this - to the knowledge of the researchers, no code has yet been written to carry out such an attack.

Wi-Fi hacking has been in the news at various points over the past year or two, most notably when a hacking expert demonstrated how easy it is to intercept email over a Wi-Fi connection at last year's Black Hat security convention.

At the time, Robert Graham, CEO of Errata Security, accessed the Gmail of a 'victim' in front of the press. What was worrying about the demonstration was just how simple it was. First, Graham ran Ferret to sniff out the packets of data on the open Wi-Fi network set up for the expo. It copied the cookies being sent across the access point. Graham then copied these into his browser with a tool called Hamster.

As he had the cookie, he could then gain password-less access to mail accounts. Graham demonstrated the methodology against different webmail providers. At that rate, we might all be using VPNs on public networks in a few years time.