A zero-day vulnerability is a vulnerability in a system or device that has been disclosed but is not yet patched. An exploit that attacks a zero-day vulnerability is called a zero-day exploit.
Because they were discovered before security researchers and software developers became aware of them—and before they can issue a patch—zero-day vulnerabilities pose a higher risk to users for the following reasons:
1) cybercriminals race to exploit these vulnerabilities to cash in on their schemes
2) vulnerable systems are exposed until a patch is issued by the vendor.Zero-day vulnerabilities are typically involved in targeted attacks; however, many campaigns still use old vulnerabilities.