Glossary
Zero-day
A vulnerability exploited or made public before the vendor has released a fix.
A zero-day (or 0-day) is a vulnerability that is exploited, or made public, before the vendor has released a fix. The name refers to the number of days the vendor has had to fix it: zero.
The term covers the zero-day vulnerability itself, the exploit that takes advantage of it and the attack that uses that exploit. Once a patch is out, people often speak of an n-day: the flaw is known and fixable, but it stays exploitable on every system that has not been updated.
Zero-days exploited in the wild often end up in the CISA KEV catalog, but many KEV entries are n-days: flaws fixed weeks, months or years earlier, which attackers exploit on systems that are behind on updates. Applying existing patches quickly remains the most effective measure.