In cyber security, a repellent is any measure that discourages an attacker from targeting a system in the first place. Unlike a control that blocks traffic after it starts, a repellent raises effort, uncertainty, or risk before an exploit attempt is worth making. Common examples include strong authentication, patched services, rate limiting, clear security banners, locked-down configurations, and visible monitoring that signals detection.
Repellents matter because many attacks are opportunistic. Scanners and phishing campaigns often move on when they meet friction, a warning, or a harder target. In defense, repellents reduce exposure and buy time for deeper controls such as logging, filtering, and incident response. They work best when they are consistent, visible, and hard for automated tools to ignore.