the weakest link
expressionsecurity
The chain proverb applied to security: the system's real strength is its most neglected component, not its best one.
Attackers do not duel your strongest control; they route around it - through the unpatched box, the reused password, the helpful human. The corollary shapes strategy: uniform decent coverage beats one magnificent wall with a gap.
Also known as: a chain is only as strong as its weakest link