A zero-day vulnerability is a software or hardware flaw that is unknown to the vendor and unpatched when attackers or researchers first discover or demonstrate it. The name comes from the fact that defenders have had “zero days” to prepare a fix.
Zero-days matter because they can bypass normal security controls until a patch, workaround, or detection rule exists. Attackers value them for stealth and reliability, especially against high-value systems such as operating systems, hypervisors, browsers, and AI software. Defenders look for signs of exploitation, reduce exposure with segmentation and least privilege, and patch quickly once a flaw is disclosed. Events like exploit contests can help security teams see which technologies are likely to attract zero-day research next.