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Perimeter

12/15/2008
03:10 PM
John H. Sawyer
John H. Sawyer
Commentary
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DNSChanger Trojan Spoofs DHCP Responses To Unsuspecting Victims

Malware analysis has been a small obsession of mine for at least the past four years. I always have a virtual machine sitting around just waiting to be subjected to the next unknown executable that lands in my lap. A psychologist might say I have some "issues" since I get excited from the thought of infecting hapless Windows machines.

Malware analysis has been a small obsession of mine for at least the past four years. I always have a virtual machine sitting around just waiting to be subjected to the next unknown executable that lands in my lap. A psychologist might say I have some "issues" since I get excited from the thought of infecting hapless Windows machines.Personally, I think it is just part of my investigative nature to need to know how something works or determine what it has done after the fact. Sometimes, when I can't get my fix doing hands-on malware analysis, I resort to reading the analysis from other analysts, like the guys behind the McAfee Avert Labs Blog.

McAfee's Craig Schmugar posted a good analysis and description of the DNSChanger Trojan's latest attack technique of spoofing DHCP responses to hosts on the local network. If you haven't heard of or had personal experience with some variant of the DNSChanger Trojan, the basic premise is that it changes the DNS entries on the infected hosts to malicious DNS servers sending unsuspecting users to a malicious site when they were trying to go to Facebook.

The newer DNSChanger Trojan variants were already extremely crafty and "evil" in that they used cross-site request forgery (CSRF) attacks to modify the average consumer routers DNS settings to affect all users on the local network. Since that attack method probably wasn't as effective as the malware authors were hoping, they've now started spoofing DHCP responses with the malicious DNS server addresses as soon as other hosts on the network around it request or renew an IP address.

Most of the blog comments I've seen in various places seem to reflect that there's no hope; in the context of the average, noncorporate end-user, there really isn't. Antivirus vendors need to do two things to combat this: develop accurate detection methods of the malware and monitor DNS settings for changes that point to malicious IP addresses. Unfortunately, the latter is the hardest because the IP addresses can change extremely fast.

Corporate-owned and managed computers have a better chance of being protected as long as their DNS settings are static, preventing any malicious changes from the spoofed DHCP responses. There is a small issue that could crop up with this protective measure because some free wireless networks I've used did not allow external DNS requests from clients. Luckily, in those situations, I've been able to use a VPN that pointed to an IP address and not a DNS entry. That's something to keep in mind if you think a VPN is the quick fix since a DNS lookup to resolve the address of your VPN server could end up being spoofed.

I really hate writing about things that end without hope, but this is one of those problems that is a result of a flaw in a fundamental technology used everywhere -- DHCP. Until it is fixed, there will continue to be attacks similar to this one, and we're going to have to rely on additional broken technologies (like AV) to try and protect us.

John H. Sawyer is a Senior Security Engineer on the IT Security Team at the University of Florida. The views and opinions expressed in this blog are his own and do not represent the views and opinions of the UF IT Security Team or the University of Florida. When John's not fighting flaming, malware-infested machines or performing autopsies on blitzed boxes, he can usually be found hanging with his family, bouncing a baby on one knee and balancing a laptop on the other. Special to Dark Reading.

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