Most organizations treat their corporate network as one homogeneous network with no internal access controls at the network level. They assume everyone on the inside of the network is a trusted “good guy.”
This is a mistake.
Employees occasionally do bad things. Often accidentally, sometimes out of curiosity, and once in a while out of malice.
Even if you trust your users not to do anything they’re not supposed to, their PCs can be compromised. If one of your users clicks on a link in an email, or opens an attachment, their PC may now be under the control of someone else. When an attacker compromises a PC through downloaded malware, they effectively become an insider. If there are no controls on the internal network, the attacker has free reign to attack any systems then they can find. This is what is meant by the common phrase “hard and crunchy on the outside, soft and chewy on the inside.” Once an attacker gets through your perimeter defenses, they become an inside user, and everything is up for grabs.
The idea behind compartmentalization is to place access controls on the inside of your network to limit communication between systems, and to identify unexpected communications that may be a sign of attack. In every network, there are systems that should never communicate with each other. Your network should enforce that separation.
A good analogy to this is a fire door. You probably have fire doors inside your building right now. Consider how a fire door works. A fire door doesn’t prevent fires, but when a fire happens, the fire door slows a fire from spreading, giving you time to escape or respond. Similarly, access controls on your inside network slow down an attacker, giving you time to detect them and respond.
- You may have lots of non-PC devices: phones, cameras, card readers, building control systems, industrial controls, etc. These systems should not be accessible from user’s PCs.
- Accounting servers should only be accessed by the accounting department.
- Printers should never access the Internet.
- Users should never access IP phone servers.
- Users should never access the console of production servers
- PCs, in general, should not access other PCs.
There are many similar rules (implicit or explicit) that should be enforced. You can do this by segregating similar devices onto VLANs, and then creating access lists on the VLAN (SVI) interface to prevent disallowed communications. As an example, put IP phones on their own VLAN, separate from user PCs and other devices. Create and apply an access list that blocks traffic between the phones and other devices.
By enforcing this separation, you can stop many kinds of attacks dead in their tracks. Moreover, if you log denied packets from your access lists, you will be able to detect violations of your policy that are a sign you’ve been compromised. An attacker will almost certainly stumble on one of these access lists, and that will tip you off to the attacker’s presence. Once you know they’re in your network, you can make an effective response.