Users of Windows laptops with Wi-Fi capabilities may be opening their hard drives up to snoopers, according to a security researcher.

A bug in the way Windows manages wireless network connections means that people using their laptops in public places are frequently giving anyone in the area the means to invade their systems, according to Mark Loveless of Nomad Mobile Research Centre (NMRC).

When a laptop has been connected to an ad-hoc network it can later begin advertising that ad-hoc network's SSID identifier without the user's knowledge, giving those nearby the opportunity to carry out an attack, according to Loveless.

He called the bug a "configuration error" rather than a security vulnerability, but gave it a severity rating of "High (albeit lame)".

"In theory all kinds of information could be siphoned off, but that depends on either a secondary vulnerability (such as MS05-039 patch not being applied), a lack of a personal firewall, or open shares. Granted, when I found a machine vulnerable to the wireless flaw, I usually found one of
those secondary issues as well, but the vast majority of laptops were not vulnerable to the wireless flaw in the first place," he said.

While the bug sounds unlikely in theory, in practice Loveless frequently came across vulnerable laptops in airports and on planes. "While visiting Charlotte, North Carolina... - walking the terminal during massive East Coast rain delays with most flights delayed by a couple of hours - I counted no less than 62 ad-hoc devices," he said in an advisory. "A conservative estimate would put half of those ad-hoc devices at risk." On four domestic US flights, he counted 11 vulnerable laptops.

"My whole point is that for the bored and idle hacker waiting for a flight or stuck on a commuter train, there are probably unsuspecting vulnerable targets nearby. And activation of the flaw actually "spreads" from vulnerable system to system, which makes it interesting," he added.

Loveless contacted Microsoft about the bug in mid-October, and the company has confirmed the issue. "As there are multiple and easy-to-implement workarounds for the issue, Microsoft has scheduled to include the fix in the next service packs," Loveless stated. Windows XP won't get another service pack until 2007, however, while Windows 2000 will not receive further service packs.

Loveless said he confirmed the bug in Windows 2000 with SP 2, SP 3 and SP 4; Windows XP Home Edition Gold; Windows XP Professional Gold; Windows XP with SP1 and SP2; and Windows 2003.

The bug is due to the way Windows allows laptops to connect to ad-hoc networks without the user's knowledge, and is made possible by the fact that most wireless networks have a small number of default SSIDs - such as "linksys", "dlink", "tmobile" and "hpsetup", Loveless said.

The situation arises when laptop "A", usually connected to a home network with an SSID such as "linksys", is switched on near laptop "B" which is set up with an ad-hoc network, also called "linksys". Laptop "A" automatically connects to the ad-hoc network of laptop "B". Then, the next time the laptop is switched on without an Ethernet connection, laptop "A" will begin advertising an ad-hoc network with the SSID "linksys", according to Loveless.

"This is basically a configuration error that spreads virus-like from laptop to laptop," he stated.

The issue is made worse by the fact that laptops with built-in wireless are usually left with their wireless connections active whether they're in use or not. An additional problem is the way Windows handles assigning IP addresses via DHCP servers, which can make it easier for an attacker to get the laptop's IP address and carry out an attack, Loveless said.

He recommended switching Windows' Wireless Network Connection settings to "Access point (infrastructure) networks only", which will disable the ad-hoc networking feature. Other workarounds include disabling wireless when not in use and using a different Wireless Client Manager.