What is a Zero Day vulnerability?

A Zero Day vulnerability refers to a flaw in software that is unknown to the software vendor at the time it is being exploited. This means that there has been no time for the vendor to develop a patch or update to address the vulnerability, hence the term "zero day" — indicating that the developers have had zero days to fix it since its discovery. This type of vulnerability is particularly dangerous because attackers can exploit it without any initial defenses in place to protect users or systems.

In contrast, the other options describe situations that do not capture the essence of a Zero Day vulnerability. A widely published and fixed vulnerability implies that the vendor has acknowledged the flaw and taken steps to remedy it, thus transitioning it out of the Zero Day category. Similarly, a vulnerability that has been public for over a year is no longer considered a Zero Day, as it is no longer unknown to the vendor. Lastly, a minor bug in software that does not affect security does not meet the definition of a vulnerability at all, let alone a Zero Day, as it does not pose any risk to data or system integrity.