A zero-day attack exploits vulnerabilities in software or hardware that are unknown to the vendor or for which a patch has not yet been released. These attacks are particularly dangerous because they take advantage of the window of time between the vulnerability's discovery and the availability of a fix, leaving systems exposed to potential exploitation. Zero-day attacks require a proactive and comprehensive approach to security, including the use of advanced threat detection systems and threat intelligence to identify and mitigate potential threats before they can be exploited.