Internal Network Access: We’re Doing It Wrong

A fundamental design flaw in network design and access management gives malware an open door into organizations.

Run the information technology clock back to the early 1980s, when universities and businesses began implementing local area networks. We connected ThinNet or ThickNet cabling to our servers and workstations and built the first local area networks, using a number of framing technologies – primarily Ethernet.

By design, Ethernet is a shared medium technology, which means that all stations on a local area network are able to communicate freely with one another. Whether devices called “hubs” were used, or if stations were strung together like Christmas tree lights, the result was the same: a completely open network with no access restrictions at the network level.

Fast forward a few years, when network switches began to replace hubs. Networks were a little more efficient, but the access model was unchanged – and remains so to this day. The bottom line:

Every workstation has the ability to communicate with every other workstation on all protocols.

This is wrong. This principle of open internal networks goes against the grain of the most important access control principle: deny access except when explicitly required. With today’s internal networks, there is no denial at all!

What I’m not talking about here is the junction between workstation networks and data center networks. Many organizations have introduced access control, primarily in the form of firewalls, and less often in the form of user-level authentication, so that internal data centers and other server networks are no longer a part of the open workstation network. That represents real progress, although many organizations have not yet made this step. But this is not the central point of this article, so let’s get back to it.

There are two reasons why today’s internal networks should not be wide open like most are now. The first reason is that it facilitates internal resource sharing. Most organizations have policy that prohibits individual workstations from being used to share resources with others. For instance, users can set up file shares and also share their directly-connected printers to other users. The main reason this is not a great idea is that these internal workstations contribute to the Shadow-IT problem by becoming non-sanctioned resources.

The main objection to open internal networks is that they facilitate the lateral movement of malware and intruders. For fifteen years or more, tens of thousands of organizations have been compromised by malware that self-propagates through internal networks. Worms such as Code Red, Nimda, Slammer, and Blaster scan internal networks to find other opportunities to infect internal systems. Attackers who successfully install RATs (remote access Trojans) on victim computers can scan local networks to enumerate internal networks and select additional targets. Today’s internal networks are doing nothing to stop these techniques.

The model of wide-open access needs to be inverted, so that the following rules of network access are implemented:

  1. Workstations have no network access with each other.
  2. Workstations have access ONLY to servers and services as required.

This should be the new default; this precisely follows the access control principle of deny all except that which is specifically required.

Twenty years ago, this would have meant that all workstation traffic would need to traverse firewalls that would made pass or no-pass decisions. However, in my opinion, network switches themselves are the right place to enact this type of access control.


One thought on “Internal Network Access: We’re Doing It Wrong

  1. TG

    I don’t know why this never occurred to me. I think this makes a lot of sense, given the “pivot potential”, if you will. Moving to a default-deny on pc-to-pc access seems like a wise move for most environments. Going now to look at the abilities of my switches….


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