Networks & Defense
Default Deny
Also: deny by default, fail closed, implicit deny
A policy where everything is blocked unless a rule explicitly allows it.
In more detail
Default deny is least privilege applied to a network or a tool. A forgotten case is blocked rather than permitted. The opposite, default allow, requires you to list every bad thing in advance, which is impossible. The common failure is a broad allow-everything rule added to fix a day-one breakage.