Network Security Fundamentals: Default Deny (UPDATED)By Mike Rothman
(Update: Based on a comment, I added some caveats regarding business critical applications.)
Since I’m getting my coverage of Network and Endpoint Security, as well as Security Management, off the ground, I’ll be documenting a lot of fundamentals. The research library is bare from the perspective of infrastructure content, so I need to build that up, one post at a time.
As we start talking about the fundamentals of network security, we’ll first zero in on the perimeter of your network. The Internet-facing devices accessible by the bad guys, and usually one of the prevalent attack vectors.
Yeah, yeah, I know most of the attacks target web applications nowadays. Blah blah blah. Most, but not all, so we have to revisit how our perimeter network is architected and what kind of traffic we allow into that web application in the first place.
Defining Default Deny
Which brings us to the first topic in the fundamentals series: Default Deny, which implements what is known in the trade as a positive security model. Basically it means unless you specifically allow something, you deny it.
It’s the network version of whitelisting. In your perimeter device (most likely a firewall), you define the ports and protocols you allow, and turn everything else off.
Why is this a good idea? Lots of attacks target unused and strange ports on your firewalls. If those ports are shut down by default, you dramatically reduce your attack surface. As mentioned in the Low Hanging Fruit: Network Security, many organizations have out-of-control firewall and router rules, so this also provides an opportunity to clean those rules up as well.
As simple an idea as this sounds, it’s surprising how many organizations either don’t have default deny as a policy, or don’t enforce it tightly enough because developers and other IT folks need their special ports opened up.
Getting to Default Deny
One of the more contentious low hanging fruit recommendations, as evidenced by the comments, was the idea to just blow away your overgrown firewall rule set and wait for folks to complain. A number said that wouldn’t work in their environments, and I can understand that. So let’s map out a few ways to get to default deny:
- One Fell Swoop: In my opinion, we should all be working to get to default deny as quickly as possible. That means taking a management by compliant approach for most of your traffic, blowing away the rule set, and waiting for the help desk phone to start ringing. Prior to blowing up your rule base, make sure to define the handful of applications that will get you fired if they go down. Management by Compliant doesn’t work when the compliant is attached to a 12-gauge pointed at your head. Support for those applications needs to go into the base firewall configuration.
- Consensus: This method involves working with senior network and application management to define the minimal set of allowed protocols and ports. Then the impetus falls on the developers and ops folks to work within those parameters. You’ll also want a specific process for exceptions, since you know those pesky folks will absolutely positively need at least one port open for their 25-year-old application. If that won’t work, there is always the status quo approach…
- Case by Case: This is probably how you do things already. Basically you go through each rule in the firewall and try to remember why it’s there and if it’s still necessary. If you do remember who owns the rule, go to them and confirm it’s still relevant. If you don’t, you have a choice. Turn it off and risk breaking something (the right choice) or leave it alone and keep supporting your overgrown rule set.
Regardless of how you get to Default Deny, communication is critical. Folks need to know when you plan to shut down a bunch of rules and they need to know the process to get the rules re-established.
Testing Default Deny
We at Securosis are big fans of testing your defenses. That means just because you think your firewall configuration enforces default deny, you need to be sure. So try to break it. Use vulnerability scanners and automated pen testing tools to find exposures that can be exploited. And make this kind of testing a standard part of your network security practice.
Things change, including your firewall rule set. Mistakes are made and defects are introduced. Make sure you are finding them – not the bad guys.
Default Deny Downside
OK, as simple and clean as default deny is as a concept, you do have to understand this policy can break things, and broken stuff usually results in grumpy users. Sometimes they want to play that multi-player version of Doom with their college buddies and it uses a blocked port. Oh, well, it’s now broken and the user will be grumpy. You also may break some streaming video applications, which could become a productivity boost during March Madness. But a lot of the video guys are getting more savvy and use port 80, so this rule won’t impact them.
As mentioned above, it’s important to ensure the handful of business critical applications still run after the firewall ruleset rationalization. So do an inventory of your key applications and what’s required to support those applications. Integrate those rules into your base set and then move on. Of course, mentioning that your trading applications probably shouldn’t need ports 38-934 open for all protocols is reasonable, but ultimately the business users have to balance the cost to re-engineer the application versus the impact to security posture of the status quo. That’s not the security team’s decision to make.
Also understand default deny is not a panacea. As just mentioned, lots of application traffic uses port 80 or 443 (SSL), and will largely be invisible to your firewall. Sure, some devices claim “deep packet inspection” and others talk about application awareness, but most don’t. So more sophisticated attacks require additional layers of defense.
Understand default deny for what it is: a coarse filter for your perimeter, which reduces your attack surface. And it’s one of the more basic network security fundamentals.
Next up, we’ll talk about network monitoring, since that is both a hot topic and fundamental to defending your network.