A single system must run a number of different network services. Among them is a web server, a mail server, a DNS server, and a MySQL database used by an e-commerce application. After several months of successful operation, a compromise is discovered: an attacker exploited a vulnerability in the mail server and ultimately obtained a root shell. However, none of the highly sensitive personal information in the e-commerce database was compromised, nor was the operation of the web server or DNS server affected. How can this be?

A.
A system administrator deployed each service in its own non-global zone.

B.
The system was deployed in a DMZ, sensibly segmented from the corporate intranet by a packet filtering firewall.

C.
An administrator had deployed IP Filter with an aggressive policy, blocking all connections to non-essential ports.

D.
Remote access to the system was only available through SSH using RSA public key authentication, protecting users’ credentials from eavesdroppers.