What is defined as a "zero-day" exploit?

A "zero-day" exploit refers specifically to a vulnerability that is actively exploited by attackers before the developers have had the opportunity to create and issue a fix or patch for the identified security flaw. This means that the developers are unaware of the vulnerability at the time it is being exploited, hence the term "zero-day," which indicates that there have been zero days available for the developers to address the issue.

These types of exploits are particularly dangerous because they can be used to infiltrate systems, steal sensitive information, or cause damage without any warning or mitigation available to the users or administrators of the affected systems. Cybersecurity professionals emphasize the importance of proactive measures to identify and address potential vulnerabilities before they can be exploited in a zero-day manner.

Other options describe concepts related to security but do not accurately define what a zero-day exploit is. A vulnerability that has been fixed by developers would not be classified as zero-day, as that indicates it has already been addressed. A new software feature released without testing pertains to development practices but doesn't convey the specific threat posed by zero-day vulnerabilities. Finally, an outdated security measure lacks relevance to the concept of zero-day exploits since the term specifically focuses on new vulnerabilities being exploited before a patch is available.