Zero-Day

A zero-day is a vulnerability that is unknown to the software vendor and for which no patch exists. The name comes from the fact that developers have had 'zero days' to fix the problem when it's first discovered or exploited. Zero-day attacks exploit these unknown vulnerabilities, making them particularly dangerous because no specific defense exists yet.

Zero-day vulnerabilities are valuable to attackers (and to some governments) because they can't be blocked by signature-based security tools that rely on knowing about specific threats. Defense against zero-days requires behavioral detection, application whitelisting, network segmentation, and defense-in-depth strategies.