State the Example of security audit as case study

Linda narrates one of her experience about a security audit. She was performing a spot audit where she was checking certain systems. It was noticed that there could be systems that was not even part of the audit. The exact location on the network layout plan was found out and it was noticed that it had an extra connection to it but she could not make out by the network map where the connection lead to. Consequently, she decided to audit that system and found that it was connected to the Internet, to that company's Intranet as well as to a customer network. Still ahead, it was found that a hacker had broken into the system and replaced the system files and put them back in proper place. Nobody in the company even knew that the system had been broken into and only two people knew that it was even connected to the Internet.