Security Mistakes That Leave You Vulnerable To Compromise #7: Failure to Compartmentalize Your Network


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.

For example:

  • 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.

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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.