What is defined as a security flaw that is unknown to the software vendor?

The concept of a zero-day vulnerability refers to a security flaw that is unknown to the software vendor at the time it is discovered or exploited. This means that the vendor has not had any time to develop and implement a patch or mitigation for this vulnerability, hence "zero days" of protection against potential attacks that leverage this flaw. Because the vendor is unaware of the issue, it poses a significant risk to users of the affected software, as they are left exposed until a solution is made available. The critical aspect of a zero-day vulnerability is its unknown status to the vendor, making it particularly dangerous for organizations that rely on the vulnerable software without realizing the security risk it represents.

In contrast, a backdoor vulnerability suggests that there is a method of bypassing normal authentication, which may be known or unknown. A known exploit is a vulnerability that has been identified and for which patches or fixes have often been deployed. A time-bomb vulnerability refers to a flaw that activates at a predetermined time, which is generally known to the developers and users to some extent. Therefore, zero-day vulnerabilities are distinctly characterized by their undisclosed nature to the software vendor, emphasizing the urgency and need for robust security measures.