The perfect is the enemy of the good
No security solution is ever perfect
Mistakes will be made, there will be unforeseen interactions between
multiple complex systems that create security holes, and there will be
vulnerabilities that aren't caught by pre-release testing
No search for mythical perfect system
Continue shipping something that is still very good and update
Deploy defenses in depth
Deploy a variety of defenses to act as a series of stumbling blocks for the
attacker.
Make it hard to get into the system, but assume that the attacker will
Put another layer of defenses in place to make it difficult to turn a user
account compromise into root or a kernel exploit
Make it difficult for an attacker to persist his presence on the system by
preventing him from adding an account, installing services, or recompromising
the system after reboot