Zero day attack

A zero-day (or zero-hour) attack or threat is a computer threat that tries to exploit computer application vulnerabilities which are unknown to others, undisclosed to the software vendor, or for which no security fix is available. Zero-day exploits (actual code that can use a security hole to carry out an attack) are used or shared by attackers before the software vendor knows about the vulnerability.

The term derives from the age of the exploit. When a vendor becomes aware of a security hole, there is a race to close it before attackers discover it or the vulnerability becomes public. A "zero day" attack occurs on or before the first or "zeroth" day of vendor awareness, meaning the vendor has not had any opportunity to disseminate a security fix to users of the software. (In computer science, numbering often starts at zero instead of one.)