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What are 0-day attacks?

The apocalyptic-sounding name of zero-day (or zero-hour) attack or threat refers to malware that exploits a yet unknown security flaw in a computer program.

As you can read in Wikipedia "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."

The name can be misleading though, because a known zero-day vulnerability might have been "known" and used for a long time before the software vendor becomes aware of it.

From a practical point of view, a software engineer who finds out such a flaw (usually in popular applications) has two options: 1. (good) contact the software vendor, describe the vulnerability and allow them to fix it and release a security patch; 2. (evil) develop a malware to exploit the flaw and gain advantage on as many computers as possible, before the software vendor finds out about the problem and closes the loop-hole.