What is a Zero-Day Attack?

A zero-day (also known as 0-day) vulnerability is a computer-software vulnerability that is unknown to those who would be interested in mitigating the vulnerability (including the vendor of the target software). Until the vulnerability is alleviated, hackers can exploit it to negatively affect computer programs, data, additional computers or a network. An exploit directed at a zero-day is called a zero-day exploit, or zero-day attack. Even after a fix is developed, the fewer the days since Day Zero, the higher is the chance that an attack against the afflicted software will be successful, because not every user of that software will have applied the solution.