Which of the following best describes a Zero Day vulnerability?

$1.$

A zero$-$day vulnerability refers to a security flaw that has been discovered by a threat actor but is unknown to the enterprise and software vendor.

$2.$

A zero$-$day vulnerability refers to a security flaw that has been discovered but not mitigated by the software vendor.

$3.$

A zero$-$day vulnerability refers to a security flaw that has been discovered but not mitigated by the enterprise.

$4.$

A zero$-$day vulnerability refers to a security flaw that is a result of a system misconfiguration.