As tends to happen from time to time, some sudden attention on the 'net (starting with the Security Fix blog at Washington Post) has been paid in the last couple days to what has been misleadingly described in some places as a "flaw" in the Windows wireless networking functionality. In reality, that's not quite the case. Rather, the potential problem (which some might argue is actually a feature) is related to an understood standard computer configuration (some would say "as-designed") of the spec governing dynamic configuration of IPv4 link-local addresses (RFC 3927 - see part 5). The authors of the spec even noted the potential risks and discussed the importance of taking that risk into consideration in design and deployment:

"The use of IPv4 Link-Local Addresses may open a network host to new attacks.  In particular, a host that previously did not have an IP address, and no IP stack running, was not susceptible to IP-based attacks.  By configuring a working address, the host may now be vulnerable to IP-based attacks." (read the spec)

Unfortunately, some have stated incorrectly that this represents an unknown or recently-discovered security hole or flaw. That's just not the case. This is, however, something that people should be aware of if they use or manage portable computers with wireless networking cards.

The problem has to do with the fact that the last wireless network name (or SSID) you successfully connected with is reused and associated with the generic IP address that gets assigned when your wireless card can't find a network to associate with, so someone who is also assigned an IP In that block and who knows what they're doing might try to connect to your computer using that network name and the generic IP address subnet. Yeah, it's technical but it's not too hard to protect yourself.

The first thing you should already have in place - and if you don't, you need to take care of this now - is a firewall to protect access to and from your computer. It's amazing how many problems can be mostly or completely mitigated with a decent and properly configured firewall. If you block incoming traffic with the firewall, then access to the wireless adapter is nowhere near as big of a deal.

On the technical side, there are a couple things that can be done to resolve the specific issue at hand. The most logical (and second most technical) step is to configure the network adapter in Windows to only allow infrastructure connections (to access points), and not Ad-Hoc connections (to other wireless cards in peer-to-peer mode). This can be done individually (on a specific computer by the user or administrator) or in a more automated fashion across a security domain (see below).

On a Windows computer, you can also get all geeked out (this is a more technical step) and disable the feature that automatically assigns the generic dynamic IP address when DHCP server is present (this auto-assign feature is sometimes referred to as APIPA - see this page for details on disabling it if interested, but use at your own risk, it involves editing the registry). It's this common and predictable IP address space that could potentially allow someone else to try to snoop into your computer, if you had none of the other standard protections - like firewalls and directory security - in place.

An even better option - where available - is to have your Windows Domain administrators control the setting for any group of computers managed by the domain's Group Policy. To do this, navigate in the Group Policy editor to:

Computer Configuration > Windows Settings > Security Settings >Wireless Networks

You notice there's nothing listed in that section by default - That's because you have to create your own policy if you want to take advantage of the features available. To do so, right click in the empty space and choose to create a new wireless policy. You'll give it a friendly name and the wizard will walk you through the steps required to set up your new policy. On the properties page (see below), you'll note an option is available to specify the network types to which you want to allow access. You can choose "Access point (infrastructure) networks only." Note that selecting this will force all computers to which the policy is applied to access point networks (so the wireless peer-to-peer networking without an access point - which is exactly the issue we're trying to mitigate - will no longer work).

Some companies use these settings to ensure the only wireless networks that business computers access are ones that are pre-approved, but that means a tradeoff between security and convenience, and road warriors often desire and need to use public access points for any of a number of reasons. How deeply and widely you apply the policies is a business decision - just be sure to consider all the potential business effects and consequences.

Note again that fixing a problem in just one place or in just one layer is most certainly not the right way to solve problems like this. Rather, taking a defense-in-depth approach, where you block access at as many layers as possible, is the way to approach network security issues.

For example, let's go back to enabling the software firewall on your computer - whether it be the Windows Firewall that is part of Windows XP SP2, or a third party firewall by a company like Symantec or others. This is another critical layer. Having a properly configured firewall in place helps to ensure access to your computer is protected, even if the wireless connection is "open." Layering protections allows you to be sure the problems are kept out, and also provides a possible mechanism to temporarily relax any one of the protections when needed in order to accomplish a specific task.