N-Day Vulnerability.

N-day refers to the number of days that have passed since public disclosure or patch availability. Although a fix is available, the world is full of unpatched assets — slow change windows, end-of-life software, embedded devices, shadow IT — leaving long tails of exposure. Attackers reverse engineer the patch ("diff" the fix) to build working exploits within hours or days; mass-exploitation campaigns then sweep the internet. N-days dominate real-world breach statistics far more than zero-days. Defenders mitigate them through fast patch management, virtual patching, asset inventories, KEV-driven prioritization, and EPSS-informed risk scoring.