What is a Zero-day?

Definition

A zero-day vulnerability refers to an exploitable weakness in software that is usually unknown to a vendor. Since this vulnerability has never been shared publicly, no days have gone by to address the issue; thus it is on “day zero”. While such a vulnerability may be intentionally introduced, more often these vulnerabilities are inadvertently created. Zero-day exploits use an exploitable weakness to carry out an attack. Such exploits can be obtained through research and investigation or purchased through private-sector providers

Read more at Woods LLP