The sandboxing appliances popularly deployed today are performing well against your average”0-day” malware threat, but capabilities decline dramatically the more targeted an adversary becomes. As such, organizations are much better at stopping the generic non-targeted “Internet threats”, but becoming more vulnerable to marginally tuned malware. For example, any piece of malware that requires the user to perform an action at a specific time (before it acts maliciously) is sufficient to evade detection in most cases.

via The Increasing Failure Of Malware Sandboxing — Dark Reading.