What are zero-day vulnerabilities?

Zero-day vulnerabilities refer to security flaws that are unknown to the vendor or developer at the time an attack takes place. This means that when the vulnerability is exploited by an attacker, the vendor has had zero days to address or patch the issue. As a result, these vulnerabilities represent a significant risk because there are no existing defenses or remediation strategies available to protect against them until the vendor is informed and releases a fix.

The term "zero-day" emphasizes not only the existence of the flaw but also the urgency and potential for exploitation before any mitigation is implemented. This is crucial for understanding the threat landscape, as attackers often seek out these vulnerabilities to gain unauthorized access or perform malicious activities as soon as they are discovered.

While other options discuss various security issues, they do not capture the core definition and implications of zero-day vulnerabilities, which specifically relate to the lack of prior knowledge by the vendor at the moment of the exploit.