A zero-day attack (also referred to as Day Zero) is an attack that exploits a potentially serious software security weakness that the vendor or developer may be unaware of. The software developer must rush to resolve the weakness as soon as it is discovered in order to limit the threat to software users. The solution is called a software patch. Zero-day attacks can also be used to attack the internet of things (IoT).

A zero-day attack gets its name from the number of days the software developer has known about the problem.