What is the flat perimeter anti-pattern, why do organisations end up with it, and what is the consequence?

The whole backbone, all servers and all clients sit in one giant perimeter with no further segmentation inside, so anyone can talk to anyone. It offers only a thin protective layer to the outside; inside, an attacker or malware can spread unhindered.

Typical reasons given:

• lower hardware costs, because fewer expensive security network devices are needed;
• lower operating costs, because fewer of them must be run;
• easier troubleshooting, because no firewall blocks anything;
• a structure that simply grew over the years;
• the felt impossibility of still capturing the real communication in the network as firewall rules.

Each reason is about saving effort, and the price is paid at the first breach: once one client is compromised, nothing stops lateral movement to every server. This is the scenario segmentation into zones (and later micro-perimeters) exists to prevent.