Open networks are tempting, especially when you really need to send or receive messages or gather some data quickly while on the road. But don't let your guard down while using open networks (such as those at at conferences, coffee shops, or hotels), or you might fall victim to an intruder. In fact, when using open networks, you should raise your guard as high as you can, which might mean deciding not to use a certain open network at all.

The decision whether to use an open network comes down to two simple questions: Do you trust that you can get on and off the network safely; and do you feel confident that your system is secure enough to withstand potential zero-day exploits?

A good example of how high the risk is happened at the 2006 ShmooCon conference. While using the conference's wireless network, a security researcher's Mac laptop fell victim to attack. Even though the researcher's laptop was secured as well as possible, the system was broken into using a zero-day exploit. Unfortunately, the presenter was not running any packet-capture tools at the time, so attempts to find out how the break-in happened were fruitless.

Another case in point occurred only last week at the CanSecWest conference in Vancouver, B.C., Canada. At the conference, an interesting challenge was presented: Break into either of two MacBook Pros running OS X and win the computer. TippingPoint (a division of 3Com) offered a $10,000 cash prize to enhance the challenge further.

Sure enough, someone broke into one of the MacBooks using a zero-day exploit against the Safari Web browser. The winning challenger, Shane Macaulay, worked with a friend, Dino Dai Zovi, who didn't attend the conference. Zovi provided the exploit, and Macaulay executed it at the conference by setting up a Mac server on the conference's wireless network. He then had one of the conference workers enter a specific URL into the MacBook's browser, which in turn connected to the server to launch the exploit. That's all that was required for the MacBook to become "owned."

The point of the latter example is that the same thing could be accomplished by a bad guy lurking on a conference network or any other open network. It doesn't matter what OS you use, the risks are basically the same. Said otherwise, zero-day exploits exist for all OSs, and it's often incredibly difficult to defend against the unknown.

If you feel you must use an open network, one way to help avoid falling victim--to some extent anyway--is to use a virtual machine (VM) configuration to perform whatever tasks you need to do. While a VM might not completely protect your system, at least when you restart the VM, its OS will come up clean, assuming of course that no one used a zero-day exploit to compromise the VM software or OS image.

Another way to possibly protect your system is to use a bootable Live CD, which you might know is basically a CD-ROM with a bootable OS. If you're interested in finding a good Live CD, head over to FrozenTech (at the URL below) where you'll find dozens that you can choose from.

While neither method I suggested is completely secure, at least both methods make it much more difficult for an intruder to "own" your computer.

As an aside, since I mentioned OS X in this column, I want to also point out that Apple released a batch of 25 security patches last week. So if you manage OS X systems, be sure to update them. You can learn more about the patches at the Apple site at the URL below.