A zero day attack, also known as a zero hour attack, takes advantage of computer vulnerabilities that do not currently have a solution. Typically, a software company will discover a bug or problem with a piece of software after it has been released and will offer a patch — another piece of software meant to fix the original issue. A zero day attack will take advantage of that problem before a patch has been created. It is named zero day because it occurs before the first day the vulnerability is known.

In most cases, a zero day attack will take advantage of a bug that neither the software’s creators nor users are aware of. In fact, this is precisely what malicious programmers hope to find. By finding software vulnerabilities before the software’s makers find them, a programmer can create a virus or worm that exploits that vulnerability and harms computer systems in a variety of ways.

Not every zero day attack truly occurs before software producers are aware of the vulnerability. Sometimes software producers learn of the vulnerability but developing a patch can take time. Alternatively, software producers may sometimes hold off on releasing the patch because they do not want to inundate customers with numerous individual updates. If the vulnerability is not particularly dangerous, software producers may choose to hold off until multiple updates are collected and release them together as a package. Still, this approach can potentially expose users to a zero day attack.

A zero day attack can be harmful to specific computers long after a patch has been created and the vulnerability has been closed. This is because many computer owners do not regularly update their software with patches made available by the software makers. To protect your system, software maker's recommend that users regularly check their site(s) for software patches, or bug fixes.

Many computer experts recommend two techniques to protect a computer system against a zero day attack. The first is enabling heuristic virus scanning, an option in anti-virus software to block currently unknown viruses and worms, because the typical zero day attack is unknown until a large number of computers have been infected. The second is to use a firewall to protect a computer against online exploits.