In the modern cyber era, attackers have proven that they can breach many organizations thought to be secured. They employ sophisticated social engineering methods and exploit 0-day vulnerabilities in order to infiltrate the target network, while bypassing defense measures such as intrusion detection and prevention systems (IDS/IPS), firewalls, antivirus programs, and the like.

http://cyber.bgu.ac.il/advanced-cyber/system/files/LED-it-GO_0.pdf