Which statement best defines a "zero-day exploit"?

A zero-day exploit refers specifically to a vulnerability in software that is unknown to the developers or vendors at the time it is discovered and subsequently exploited by attackers. This term emphasizes the critical nature of such vulnerabilities because there are zero days of protection in place — the vendors have had no time to address the flaw since they are unaware of its existence.

This type of exploit is particularly dangerous because attackers can take advantage of the vulnerability before any patch or remediation is available, leaving users exposed and organizations at risk until the vendor can develop a fix. Understanding this definition is crucial in the context of secure software design, as it highlights the importance of proactive security measures and robust testing to identify potential vulnerabilities before they can be exploited.