What is a zero-day vulnerability?

A zero-day vulnerability refers to a flaw or weakness in software that is not known to the vendor at the time it is discovered by attackers. Since the vendor is unaware of the vulnerability, they have had "zero days" to address or patch the issue, making it particularly dangerous. Attackers can exploit such vulnerabilities to gain unauthorized access, disrupt services, or steal data before any mitigative actions can be taken by the vendor or users.

The urgency and risk associated with zero-day vulnerabilities lie in their stealth; they can remain undetected until they are leveraged in an attack, often causing significant damage. This concept underlines the importance of proactive security measures and continuous monitoring to identify potential vulnerabilities before they can be exploited.

In contrast, other answer choices address situations or categories of vulnerabilities that differ; for example, a vulnerability that has been publicly disclosed but not yet patched indicates awareness and a potential timeline for remediation, rather than being entirely unknown. Similarly, security risks tied to outdated software can involve known vulnerabilities for which patches exist, while the mention of exploitation limited to certain days suggests a misunderstanding of how vulnerabilities function.