A couple weeks ago I posted a blog article with some sample file hashes and domain names associated with the recent Google hacks (think APT or Aurora).

Since then I’ve had quite a few people ask me, if you have a system that you suspect might have been compromised, how do you search that system for files that are malicious if you have a list of hashes that you know are malicious? In other words, you have a list of hashes and you want to know if there are any files on your file system that has the same hash value.

Disclaimer – before we continue you should know, hashes of malicious files are just one way of attempting to discover if your system has been compromised. Especially when dealing with a threat like APT, which is highly intelligent and adaptable, you have to know that if the threat knows that you’re on to them and that you’re looking for a specific set of hashes, that they’re smart enough to adapt. What will they do, they’ll change their malicious files so the hashes change as well. There’s no doubt this is a limitation. But utilizing the technique we’re about to describe you can at least start to eliminate some of the low hanging fruit. You may also want to investigate the projects involved with fuzzy hashing. This may be an alternative to some of the standard techniques described here.

Ok, now that you’re ready to start examining your systems for malicious files, here is a process to consider:

Step One: Assemble a Text File of Known Malicious Hashes. The first step you need to follow is to gather a list of hashes of known malicious files. This will be the list of hashes you’re scanning your system for. Remember, the value of your scan will only be as good as the list of hashes you have. A starter list of MD5 hashes is currently being hosted at Enclavesecurity.com and can be found here if you’re looking for a list to get you started. This list certainly is not comprehensive, but at least is a place to consider building your first list from.

Step Two: Decide Which Hashing Tool to Use. There are a number of good tools that you can use to scan your system and to generate hashes of all the files on your file system. Many of these tools are commercial and there are open source tools for this as well. On the commercial side tools like Tripwire, Lumension, and Bit9 are quite effective at this. There are certainly others, but many of you are already using these tools, so you might as well take advantage of them. Unfortunately there are also many of you that simply cannot afford these tools. If you’re looking for a good open source tool to use to start scanning your systems, let me recommend MD5Deep. This is a tool in the public domain that is especially useful for this purpose. While there’s not enough time in this post to talk about how to use the tool, we’ll post more on it later. You could also consider rolling your own scripts, using PowerShell or shell scripting to generate these hashes as well (but I still recommend MD5Deep – it’s cross platform, supports recursive file scans of directories, and natively interfaces with a number of hash databases).

Step Three: Scan Your System. Now that you have your list of hashes for malicious files and you have your scanning tool, now it’s time to scan your system to see if any files with these hashes exist on your system. This is the basic part of the exercise – do you have malicious files on your system or not? Depending on the tool you’re using this process will be slightly different, but in the end you’re trying to determine if you have a compromised host. Auditors – you should be asking companies the control question, if law enforcement approaches you with a list of hashes like we’re describing here and they say you need to check your system to see if any of these files exist on particular hosts in your environment, how would you look for the hashes? Ask to see their process in action (we want more than tabletop reviews here).

Step Four: Automate System Scans. Finally once you have your tool working in a manual mode, automate the scans. This is one of the major principles of the 20 Critical Controls / Consensus Audit Guidelines that we talk so much about. Manual scans are fine when you need to use them – but how much better is it if you could implement a tool that would be constantly scanning your systems and would notify you one of the hashes were discovered? Automation is key.

While there are certainly other ways to go about looking for malicious files on your file system or indicators of compromise on a system, examining file hashes certainly has to be part of your arsenal. If you’re auditing a system, knowing that you have a control in place to scan for signatures of known bad files has to be part of your toolkit. Traditionally we’ve done this with anti-malware tools, but unfortunately many of the large anti-malware vendors still don’t let you know which hashes they’re scanning for and they don’t give you the ability to add hashes that you’d like to scan for in their tools. Thus we’re left to our own devices to discover if files with these signatures are still on our systems.

Hopefully putting this tool in your toolkit gives you one more angle to consider when looking for indicators of a compromise on your systems.