Ashar Aziz, founder of FireEye, recognized early on that custom malware, zero day threats, and drive by downloads could all slip by common defenses that most organizations have deployed (firewals, IPS, AV).

FORBES: Modern malware requires modern methodologies to block

Custom Trojans are as hard for signature based anti-virus to catch as the dreaded zero-day threats which exploit previously unknown vulnerabilities.

FORBES: Modern malware requires modern methodologies to block

While some customers use this to overcome problems they have with enforcing vulnerability and patch management policies it is a cumbersome technology that does nothing to address new threats, zero day vulnerabilities, or insider attacks.

FORBES: Sophos + Astaro: Good companies, bad deal