A zero-day exploit is a type of cyber attack that targets a software vulnerability that is unknown to the software vendor or to antivirus vendors. A zero-day vulnerability is a flaw in a computer system that was previously unknown to its developers or anyone capable of mitigating it). Until the vulnerability is mitigated, threat actors can exploit it. An exploit taking advantage of a zero-day is called a zero-day exploit, or zero-day attack). Zero-day attacks are especially dangerous because the only people who know about them are the attackers themselves. Once they have infiltrated a network, criminals can either attack immediately or sit and wait for the most advantageous time to do so. Zero-day vulnerabilities can take multiple forms, such as missing data encryption, missing authorizations, broken algorithms, bugs, problems with password security, and so on, making them challenging to detect. Once a patch is written and used, the exploit is no longer called a zero-day exploit.