How to spoof a MAC address

MAC address filtering for wireless networking isn’t real “security”. Anyone who pays any attention to current trends in wireless security at all should know that MAC filtering is less effective than WEP — and that WEP can be cracked almost instantly these days with commonly available tools.

This doesn’t mean MAC filtering is useless. Its resource consumption is almost unmeasurable, and even if it doesn’t keep out any reasonably knowledgeable security crackers willing to spend a few moments gaining access, it does keep out a lot of automated opportunistic attacks that are aiming solely for the absolute lowest-hanging fruit on the security tree. Since that lowest-hanging fruit consists of the majority of wireless access points, MAC filtering can be of value as a way of turning away the majority of opportunistic attackers.

Don’t rely on MAC filtering alone, however. Please, just don’t. It’s a bad idea. People seem to think “Oh, well, sure a determined attacker can get past it, but not anyone else.” It doesn’t take much determination at all to spoof a MAC address. In fact, I’ll tell you how:

1. “Listen” in on network traffic. Pick out the MAC address. This can be done with a plethora of freely available security tools, including Nmap.
2. Change your MAC address.

You can spoof a MAC address when using Nmap with nothing more than a --spoof-mac command line option for Nmap itself to hide the true source of Nmap probes. If you give it a MAC address argument of “0”, it will even generate a random MAC address for you.

For more general MAC address spoofing, your MAC address is trivially reset with tools available in default installs of most operating systems. Here are some examples:

• Linux: ifconfig eth0 hw ether 03:a0:04:d3:00:11
• FreeBSD: ifconfig bge0 link 03:a0:04:d3:00:11
• MS Windows: On Microsoft Windows systems, the MAC address is stored in a registry key. The location of that key varies from one MS Windows version to the next, but find that and you can just edit it yourself. There are, of course, numerous free utilities you can download to make this change for you as well (such as Macshift for MS Windows XP).

All of these techniques can of course be automated by self-propagating malware, and the creation of the malware can even be automated to some extent by existing malware creation “kits”. If that doesn’t convince you that MAC filtering does not provide real security, I don’t know what will.