A zero-day attack is an attack that is still unknown to the vendor, software developer, and/or system administrators. It is an attack that is newly discovered by malicious users.
A zero-day (also known as zero-hour or 0-day) vulnerability is an undisclosed computer-software vulnerability that hackers can exploit to adversely affect computer programs, data, additional computers or a network.
It is known as a "zero-day" because it is not publicly reported or announced before becoming active, leaving the software's author with zero days in which to create patches or advise workarounds to mitigate its actions. In effect, zero time has passed since the exploitable bug's existence …