Cyber Dictionary
Zero-day
A software vulnerability that is exploited before the vendor knows about it — leaving zero days to ship a patch.
A zero-day (or 0-day) is a flaw that attackers find and weaponize before the people who could fix it are even aware it exists. The "zero" is the number of days the defender has had to prepare.
Because there's no patch yet, prevention isn't an option — only resilience is:
- Defense in depth so one failure isn't game over.
- Detection that assumes the perimeter has already been breached.
- Fast patching for the moment a fix finally lands.
This is why "we are 100% secure" is always a punchline.