Definition
A zero day refers to a previously unknown security flaw in software or hardware that is discovered by attackers before the vendor is aware of it. This vulnerability remains unpatched and unmitigated, making systems susceptible to exploitation. Attackers can leverage zero days to gain unauthorized access, steal data, or disrupt operations before any defensive measures can be implemented. For instance, a newly discovered exploit targeting a popular web browser that allows remote code execution without user interaction is a classic zero-day scenario. Unlike known vulnerabilities that have patches available, zero days present an immediate and critical threat because there is no immediate fix. Understanding zero days is crucial for cybersecurity professionals and appears on exams like CompTIA Security+, CySA+, and CISSP.