Squashing Malware With Snort In-Line

Snort is a powerful open source intrusion detection system (IDS). What surprises me is how many security people have never touched it to learn more about how IDS works -- or how easy it is to evade many IDS signatures that are designed to look for known bad traffic.There is functionality within Snort that allows it to be used as an IPS and block traffic instead of just notifying when a signature was matched. When combined with rules from the Emerging Threats project and the "160 Day Rule Set" from the SRI Malware Threat Center, Snort in in-line mode can block attacks, malware downloads, botnet command and control (CnC) communications, and more.

Let's take a closer look at some of the rules. The quickest way to get a feel for what rules are useful is to review the list of Snort signatures listed on SRI's "Most Effective Malware-Related Snort Signatures." For the sake of focusing primarily on malware, I'm going to avoid the noop rules, which are more focused on attacks. If you want to know more about what the noop rules are detecting, then see the Wikipedia article on Buffer Overflows and the section on NOP sleds.

The top 2 and 3 rules from the SRI list are executables being downloaded from a remote host. I mentioned in my Tech Insight on Friday that if your users do not need to be installing software on their desktops (which they shouldn't be -- they're users), then they should not be allowed to download executable files. These are two rules that could be put into block mode except for authorized management desktops and would be quite effective at blocking a lot of malware that is currently using HTTP and other unencrypted communication channels.

I got a kick out of seeing the rules detecting TFTP activity. Wow! Really? I think the 9'0s called and want its post-exploitation techniques back. I know I'm joking about it, but TFTP is still used during attacks against Windows systems. The number of people who actually use TFTP from a Windows system for legitimate purposes has to be small. During the hardening process on your systems, get rid of "tftp.exe" and implement a block rule with Snort because it's highly unlikely that you need any TFTP traffic to traverse your network to/from the Internet.

One more quick one. Do you ever take a look at IRC traffic in your network? It's surprising that botnets are still using unencrypted protocols like HTTP and IRC, but sure enough, if you take a look at some of the effective rules, you'll see about a half dozen IRC-related signatures in there. If you're in an environment that doesn't have a business need for IRC, then dump it. Snort serves as a great learning tool, but works quite well at fighting bots, fake antivirus, and other malware. But, as with any blocking technology, using Snort inline requires tuning. There's no way you can just drop it inline and expect it to not block legitimate traffic. Run Snort inline with only alerting to begin with (or out-of-band) to get a feel for what's going on within your network and what would be blocked. Try to determine whether there are any false positives, make exceptions where necessary, and remove any rules that could be too harmful to production traffic. In the end, you will end up with another powerful layer for stopping malware.

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.