Complexity is the hidden set of constraints, dependencies, and tradeoffs that make a simple idea harder to build safely. In cybersecurity, complexity appears when a system has many moving parts: services, permissions, updates, interfaces, and trust relationships. The more parts that interact, the more chances there are for misconfiguration, bugs, and unexpected behavior.
It matters because attackers often target complexity itself. They look for weak defaults, forgotten components, conflicting policies, or chainable flaws that only exist in a complicated environment. Defenders face the same challenge: secure design must balance usability, performance, and reliability without creating gaps. Good security work tries to reduce unnecessary complexity, document required exceptions, and test how systems behave in real conditions. In practice, complexity is often where simple assumptions break down and where both attacks and defenses succeed or fail.