What is Zero-Day?

Zero-Day, explained

A zero-day vulnerability is a flaw that attackers can exploit before the software maker has released a patch. An attack using such a flaw is called a zero-day exploit. Because no fix exists, traditional defenses that rely on known signatures may not detect it.

Zero-days are valuable to criminals and intelligence agencies alike, and they are sometimes bought and sold on gray or black markets. Responsible researchers instead report them to vendors, sometimes through bug bounty programs, allowing time for a fix before public disclosure. Defenses include behavior-based detection, quick patching once fixes are released, reducing the attack surface, network segmentation, and applying the principle of least privilege so a single exploit causes less damage.