PCI-DSS Segmentation with Host-Based Firewalls

One of the frequent queries we have faced in the past months as we ramp up our consultancy and advisory for travel agencies and other merchants, has been the question of segmentation.

Now, before travel agencies were imposed with the requirement for PCI-DSS by IATA, we had very few opportunities to work with small merchants for PCI-DSS. It’s not because small merchants are exempted from PCI. They are not. Small merchants must be PCI compliant, but in reality, very few banks are chasing smaller merchants for their compliance. Our experience with merchants had been with the fairly large ones – the large petrol companies, the large retailers, the telcos and the largest travel agency being our experiences. From the time we started PCI back in 2010 to around 2014, it has mainly been for financial institutions and banks. But now with IATA flexing their regulatory muscle to make sure agencies are PCI compliant by 1st of March 2018, we have had plenty of opportunities to go into much smaller environments that we are used to. And it has been a really great experience.

So when we discuss about the topic of network segmentation, we need to be clear from the start:- it’s actually NOT a PCI-DSS requirement. PCI doesn’t state that we need to segment our network. We could very well be PCI compliant on a flat network. Page 11, of PCI-DSS v3.2 states so:

“Without adequate network segmentation (sometimes called a “flat network”) the entire network is in scope of the PCI DSS assessment.”

And we have done this before. One of our client has a completely isolated network for PCI-DSS with its own gateway and basically its a flat network with everything as CDE (Card Data Environment). Possible, but in enterprise environment, probably not so realistic if it drags in hundreds of systems. Without going too much into scoping, the main topic of this article is: if we need to segment, how do we do it?

At the onset, the question seems superfluous. How to segment? Why, by network subnets of course, or by VLANs (virtual LANs). These terms (subnet and VLAN) have been used interchangeably by myriad of customers over the years, and in most cases, they actually do multiple VLANs across different subnets, but in theory you can also have VLANs on single subnet as well. So, no – VLANs and subnetting are actually not the same but for the sake of not being pedantic, most of the time, we just allow the client to use whichever term they choose.

In most cases over the years, our clients won’t have a problem with this. Segmenting either via VLAN or network subnet, they can achieve this fairly easily through their switch or their edge router, as they usually have advanced firewalls/routers/L3 switches deployed in their network.

But going into the very small companies with a handful of people, no technology personnel, and running the D-Link DIR-615 low end routers provided by Telekom? How do we do this?

We have heard other consultants declare that these companies need to invest in enterprise grade firewalls/routers to achieve PCI compliance, because some of the entry level router/firewalls are unable to do any segmentation or VLAN. Of course, you could hack the DIR-615 to WRT and that might provide you some limited VLAN capability, but that’s beyond the scope of this article. And in any case, we doubt any of the smaller merchants have the inclination to fiddle around with their routers. So if you are stuck with a firewall/router that cannot do any network segmentation, does that mean that everything needs to be brought into scope? Does that mean you need to spend thousands to get a firewall upgrade?

So let’s have a couple of references here. First of all, the canon document from PCI will help, this is the official PCI-DSS v3.2 documentation, page 11, stating a few salient points:

Without adequate network segmentation (sometimes called a “flat network”) the entire network is in scope of the PCI DSS assessment.

This phrase actually enables many people to pre-suppose that PCI is stating that the only segmentation allowed here is by the methods we discussed above – i.e anything that creates a non-flat network. But this is confusing because when we say ‘flat network’, we are already indicating we are referencing to Layer 3. However it’s entirely possible to have layer 2 VLAN isolating systems within the SAME SUBNET (multiple VLANs – Single Subnet design). Heck, you could even have multiple subnet on a single VLAN if you want … I think I remember this from my Cisco CCNP days. So, actually, in theory , unless PCI refers to something else when it says ‘Flat Network’, their statement isn’t that accurate. You could isolate systems in a flat network.

Network segmentation can be achieved through a number of physical or logical means, such as properly configured internal network firewalls, routers with strong access control lists, or other technologies that restrict access to a particular segment of a network.

While agreeing on this one as a whole, the other confusion here is the term “Physical OR logical”. As tech nerds, we take these conjunctions very seriously. For instance,  my wife asked me the other day if I wanted a cheeseburger OR a double quarterpounder happy meal. The answer to that would be “TRUE”, meaning, Yes, I can have cheeseburger OR a double quarterpounder since “OR” here is inclusive. As long as any or both of those statements are true, it’s true.  This is usually what we do in Boolean values, for instance

1 > 2 || 3 > 2 = TRUE

1 > 2 && 3>2 = FALSE

So back to the phrase Physical OR logical, this generally means PCI accepts Physical segmentation, even if there is NO LOGICAL SEGMENTATION? What does that mean? Does it mean if I have two systems hooked into the same switch, on the same network, pinging each other, I set up a physical brick wall between these two systems, I have achieved Network Segmentation? Surely not. The physical segmentation example here would be having two separate switches servicing two different networks as opposed to using a single switch and using it’s logical functions to achieve that segmentation. Can both be used or one or the other? Yes, either can be used. So whoever have written this phrase either needs to clarify this statement proper, or simply, he or she is !(Tech Nerd).

At a high level, adequate network segmentation isolates systems that store, process, or transmit cardholder data from those that do not.

So finally they decide and say, ok, anything that ISOLATES systems can be considered network segmentation. So at least we have a lead here to go with. Anything that ISOLATES.

The next journey we take is to this document:


Section 3.1, page 13:

Examples of controls that could be applied to prevent out-of-scope systems from compromising a connected-to or security-impacting system include:

– Host-based firewall and/or intrusion detection and prevention system (IDS/IPS) on in scope systems that block connection attempts from out-of-scope systems.

This is one indication that PCI looks at alternate ways of ‘segmentation’, other than getting an enterprise grade network firewall. Once more, the conjunction used here is “AND/OR”, which we take to mean, either AND (&&) or OR (||) can be used for these two statements (Host-based firewall, IDS/IPS). So what this basically states is that a host-based (not network firewall) firewall is good enough, if configured properly to be considered as a segmentation tool.

Now if you do know a little history behind this documentation, it has a grandfather document called “Open PCI DSS Scoping Toolkit”, a copy can be found here:


This was way before the PCI-DSS document came about. We had to use the OPEN PCI scoping toolkit to define what is in scope, not in scope, CDE, non-CDE in scope etc. This is why sometimes we say systems that are non CDE are ‘infected’ , i/e pulled into scope because they are in the same subnet/VLAN. This term isn’t found in the PCI document but is used in the old scoping toolkit document. Of course, this document is deprecated and the SSC doesn’t officially endorse it. However, some concepts had made its way into the SSC scoping document and what we are focusing here is mainly on the usage of host based firewall, and whether it’s logical for it to be used for some sort of segmentation. Other parts of this document has been succeeded by the official PCI scope document, so be aware. Back to this document, a few QSAs had looked at us in amusement when we used these terms and some even commented that these are very strange terms we are using, showing how young these QSAs actually are. I am not sure about the other regions, but I have had discussions with QSAs who are 10-15 years younger than me and never had one day of experience in actual security operations. One QSA even insisted we put our logging system into the DMZ as good security practice, which I then responded with an emoji face slap to our customer. With all due respect to QSAs, I have had many arguments with them over the years – some are very good, very experienced; while some are, as Bart Simpson would put it: “Meh.”

Anyway, we digress.

In the scoping toolkit, Page 13 gives an indication of what we are talking about:

The mechanism providing the isolation or controlled access functionality may be either logical or physical. Examples of mechanisms include network and host-based firewalls, virtual routing and switching appliances, and access control lists

This is still less clear due to our “AND” and “OR” arguments, because aside from the illogical “logical or physical” statement (which PCI clearly inherited), we have the problem stating “network and host-based firewalls, virtual routing and switching appliances, and access control lists”. This, to us, might mean we need ALL of these things for isolation to be TRUE.

Thankfully, this is clarified further down in Page 36:

In order to restrict other workstations on the same network from being “infected,” the dumb terminals must be isolated (e.g., using a host-based or network-based firewalls, etc.).

The example here is “using a host-based or network-based firewalls.”. As you now are very well aware, this means this statement is true if any of these options, or both these options are true.

You see, some writers do not think twice about the usage of “AND” and “OR” operators or ‘conjunctions’ to normal English-speaking people. These are extremely powerful operators and carry entirely different meanings to what normal people may deem as normal sentences having the same meaning. Another key life example here would be if your wife (again a very relevant example) were to ask you after a late night out with the guys whether you’ve been to the bar to watch football or to watch strippers, to which you respond: “YES”.

So be careful because different people parses sentences differently, depending on whether you see life in code or not. It could very well change your life.

We have also discussed this topic of segmentation at length with some senior QSAs (QSAs who have much more experienced compared to the green horns) and they have agreed that host-based firewall, or Host IDS are acceptable forms of isolation, but requires a significant amount of configuration to ensure isolation is done properly. “Done properly” here carries a fairly subjective weight to it. QSAs are a funny lot, because many of the requirements in PCI are general, and then it’s up to the QSAs to decide whether a particular control satisfies their own concerns whatever that might be. To summarise, segmentation can be carried out easier through deployment of a network firewall and getting the segmentation rules sorted out there, but if the merchant is short on funds, and have 1 or 2 systems only to configure, a fix could be a “properly configured” host-based firewall, or a host-based IDS/IPS.

Segmentation testing still needs to occur, though, but that will be for another article for another day.

Now, I will have my coffee OR tea to finish up my day. TRUE.

For more information on PCI-DSS, feel free to drop us an email at pcidss@pkfmalaysia.com.


  1. Mark Haller

    I’ve been unable to convince others at my company that a host-based firewall provides adequate segmentation. The segmentation guide provides some pretty clear points as it uses the term “network segmentation”, which is to mean classical definitions involving VLANs, subnets, etc.

    Particularly if you look at “FIGURE 1 – PCI DSS Scoping Categories” (page 10 ) it states a system is “In Scope” if “System component is on the same network segment (for example, same subnet or VLAN) as systems that store, process, or transmit cardholder data.” This seems to clearly exclude the idea of host-based firewalls, as I believe that if any packet hits the host is it not segmented in their view.

    I think PCI is well out of date to define segmentation this way. There are other types including host and hypervisor that are likely as secure, and misconfiguration is common with trying to build up complex network based segmentation solutions. I also think it is worth noting that some companies – Illumnio, and Color Tokens – seem to be providing solutions that built on host-based microsegmentation technology. While these are more robust than a travel agency is going to buy into, I do wonder if there is a shift under foot.

    Also, if you consider the COVID-19 pandemic and a large amount of telework happening, it would seem that host-based firewalls, IDS, and monitroing along with zero trust would be a way to ensure segmentation on a home network, similar to what you’ve outlined in this post.

    Can you speak to any of the points above?

    • pkfavantedge

      Thanks for weighing in. In our experience, most QSAs may not accept host-based firewall when they do a level 1 compliance program with our clients. When we wrote this article, we were specifically looking at very small merchants, doing virtual terminals from their homes, no inbound services, dynamic IP broadband, limited number of devices and doing self assessment questionnaires (SAQ), without storage of card data and utilising PCI compliant virtual terminal solutions (GDS). In our opinion, as PCI has also given examples of host-based firewalls as possible controls, and the absolute impracticality for home-based users to go out and buy a firewall (at least in our country where the average salary is around USD800 per month), at that point, a host based firewall properly configured would deem to provide a fair assurance for segmentation. But it was specific to the environment and risk. We won’t be able to pass this in a financial institution, for instance. As an aside, even AWS wrote that their implementation of Security Groups are equivalent to host-based firewalls and QSAs have been known to accept security groups as proper segmentation of CDE/Non-CDE.

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