Perimeter is intended to control and scrutinize access.
Now, systems are interconnected and standalone system is no longer considered effective. This will then expose the attack surface.
Example is port 80. You have web site for point of presence in the market. Web site needs to allow anonymous access, or the Internet surfer. Though firewall is deployed, the web port (TCP 80, 443 or whatever is required) must be opened. Attack then shifts to application like injecting malicious contents passing thru the network layer, submitting large amount of requests to slow down or corrupt the system, manipulating client side data and resubmit to back-end.
Counter-measures will then require
- Software Secure Development Lifecycle
- Secure configuration
- Regular security patches and upgrades
- Periodic comprehensive assessment (indeed, some industries mandate this)
- Situation awareness for different types of roles involved