Brittleness is a tendency to break, crack, or fail when only a small amount of stress or change is applied. In cyber security, the term is often used for systems, controls, or workflows that work well only under ideal conditions but collapse when an attacker, configuration change, or unexpected input pushes them outside that narrow range.
Brittle software and defenses are dangerous because attackers look for edge cases. A brittle authentication flow, a hard-coded path, a fragile detection rule, or a legacy protocol can fail in ways that create outages or open a path for exploitation. Security teams try to reduce brittleness with layered controls, graceful error handling, patching, and testing under abnormal conditions so that a small disruption does not become a full compromise.