What is a zero-day vulnerability? 

0-Day vulnerabilities are a vulnerability in the wild, without any vendor patch. The zero means “before the patch exists.” Typically, 0-days are discovered before good security researchers and developers are aware of them—and before they can issue a patch—zero-day vulnerabilities pose a higher risk to users for the following reasons. First, cybercriminals race to exploit these vulnerabilities to cash in on their schemes. Second, vulnerable systems are exposed until a patch is issued by the vendor and the patch is installed by all the vendor’s customers.