Trust boundary

A trust boundary is the point where something crosses from somewhere you do not control into somewhere you do. A form filled in by a stranger arriving at your website. A message passing from one part of a company to another.

That crossing is where you should be checking. Two mistakes are common. One is checking something as it arrives, then trusting it forever after. The other is assuming anything already inside the building must be fine. If you only check at the front door, then anybody who gets through the front door is trusted everywhere.