A while back, I posted this question on Twitter.
I realize that Twitter is a difficult medium to articulate full discussions, so I wanted to engage the topic with a blog post. Over the last couple years, I have focused a fair amount of time drawing attention to the use/misuse of trusted binaries to circumvent Application Whitelisting (AW) controls. What I have not often discussed, is the actual effectiveness that I have seen of using AW. I would like to take the time to describe what I see are the strengths of AW, and encourage organizations to consider if it might work for their environments.
The genesis of this discussion came from my colleague, Matt Graeber (@mattifestation). We’ve spent a fair amount of time looking at this technology as it applies to Microsoft’s Device Guard. And while we agree there are bypasses, we also believe that a tool like Device Guard can dramatically reduce the attack surface and tools available to an adversary.
One question you must ask yourself and your organization is this… How long will you allow the adversary to use EXE/DLL tradecraft to persist and operate in your environment? I have heard a great deal of discussion and resistance to deploying AW. However, I personally have not heard anyone who has deployed the technology say that they regret whitelisting.
When the organization I used to work for deployed AW in 2013, it freed up our team from several tasks. It gave us time to hunt and prepare for the more sophisticated adversary. There are many commodity attacks and targeted attacks that take various forms. However, one commonality they all often share is to drop an EXE or DLL to disk and execute. It is this form of attack that you can mitigate with AW. With whitelisting, you force the adversary to retool and find new tradecraft, because unapproved, unknown binaries will not execute…
How long will you continue to perform IR and hunt C2 that is emitted from an unapproved PE file?
Here are some of the common reasons I have heard for NOT implementing AW. There are probably others, but this summarizes many.
1. Aren’t there trivial bypasses? It doesn’t stop all attacks.
2. Too much effort.
3. It doesn’t scale.
I’ll take each of these and express my opinion. I’m open to dialogue on this and if I’m wrong, I would like to hear it and correct course…
1. Aren’t there trivial bypasses to AW? It doesn’t stop all attacks.
There are indeed ways to bypass AW. I have found a few. However, most of the bypasses I have demonstrated require that you have already obtained access to, and have the ability to execute commands on the target system. How does the attacker gain that privilege in the first place if you deny them arbitrary PE’s? Most likely it will be from a memory corruption exploit in the browser or other application. How many exploit kits, macros, or tools lead to dropping a binary and executing it? Many do…
Most of the bypasses I have used are rooted in misplaced trust. Often administrators of AW follow a pattern of “Scan A Gold Image & Approve Everything There”. As Matt Graeber has pointed out to me several times, this is not the best approach. There are far too many binaries that are included by default that can be abused. A better approach here is to explicitly trust binaries or publishers of code. I can’t think of a single bypass that I have discovered that can’t be mitigated by the whitelist itself. For example, use the whitelist to block regsvr32.exe or InstallUtil.exe.
Don’t fall victim to the Perfect Solution Fallacy. The fact that AW doesn’t stop all attacks, or the fact that there are bypasses, is no reason to dismiss this as a valid defense.
“Nobody made a greater mistake than he who did nothing because he could do only a little.” –Edmund Burke
AW, in my opinion, can help you get control of software executing in your environment. It actually gives teeth to those Software Installation Policies. For example, it only takes that one person trying to find the Putty ssh client, and downloading a version with a backdoor to cause problems in your network. For an example of how to backdoor putty see this recent post. Or use The Backdoor Factory (BDF). The thing is, it doesn’t matter that putty has a backdoor. The original file has been altered, and will not pass the approval process for the whitelist, and the file will be denied execution. Only the approved version of putty would be able to execute in your environment.
2. Too much effort.
Well… I’ve heard this, or some variation of it. I understand that deploying and maintaining AW takes tremendous effort if you want to be successful. It actually will take training multiple people to know how to make approvals and help with new deployments.
You will actually have to work very closely with your client teams, those in IT that manage the endpoints. These partnerships can only strengthen the security team’s ability to respond to incidents. You can leverage tools like SCCM to assist with AW approvals and deployments.
The level of effort decreases over time. Yes, there will be long hours on the front end, deploying configurations, reviewing audit logs, updating configs, etc… Some admins are so worried they will block something inadvertently; they are paralyzed to even try. I think you’ll find out, Yes, you will block something legitimate. Accept that this will happen, it’s a learning process, take it in steps. Use that as an opportunity to get better.
I’ll say it again; I haven’t met anyone who has made the effort to deploy AW say that they regret the decision…
If you think it’s too hard, why not try 10% of the organization and see what you learn?
Stop telling me you aren’t doing this because it’s too hard… Anything worth doing well is going to require some effort and determination.
3. It doesn’t scale.
Nope, it may not in your environment. I never said it would… You must decide how far to go. You may not get AW everywhere, but you can still win with it deployed in critical locations. The image below describes how I think about how AW applies to different parts of your organization. It is not a one-size-fits-all solution. There are approaches and patterns that affect how you will deploy and configure whitelists. I think you should start with the bottom, and work your way up the stack.
Start to think of your environment in terms of how dynamic the systems are. At the low end of the are those fixed function systems. Think about systems similar to Automated Teller Machines. These often only need to be able to apply patches. New software rarely ever lands here. Next, you have various department templates, each department will be unique, but likely fits a pattern. Then IT Admins, who often need to install software to test or have more dynamic requirements. At the top of the environment, are Developer workstations. These are systems that are emitting code and testing. I’m not saying you can’t whitelist here. You can, I’ve done it. But it will require some changes to build processes, code signing etc…
Yes, this is an overly simplified analogy, but I hope it helps you see where you can begin to prioritize AW deployments.
So, begin to reorient how you think about your systems to how dynamic they are. You will have your quickest wins and earliest wins by starting at the bottom and moving your way up the hierarchy.
I am curious for open debate here. If AW sucks, then let me hear why. Tell me what your experience has been. What would have made it work? I’m interested in solutions that make a long term actual difference in your environment. It is my opinion that AW works, despite some flaws. It can dramatically reduce the attack patterns used by an adversary and increase the noise they generate. I also believe that by implementing AW, your security teams can gain efficiencies how they operate. I am open to learn here.
If you are tasked with defending your organization, I’m asking you, as you begin to roll out Windows 10, to consider using Device Guard.
Ok, that’s all I have today. Sincere feedback welcome. If you think I’m wrong, I’d like to hear why...