A zero-day vulnerability is a security flaw in software, hardware, or firmware that is unknown to the party responsible for patching or fixing the flaw. Until the vulnerability is mitigated, threat actors can exploit it, and an exploit taking advantage of a zero-day is called a zero-day exploit or zero-day attack). Zero-day vulnerabilities pose a higher risk to users because they were discovered before security researchers and software developers became aware of them and before they can issue a patch. 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 exploits are difficult to defend against because they are so difficult to detect. Antimalware software, intrusion detection systems (IDSes), and intrusion prevention systems (IPSes) cant recognize the attack signature because one doesnt yet exist. Since, by definition, a zero-day vulnerability cant be known in advance, there is no way to guard against a specific exploit before it happens. However, there are some things that companies can do to reduce their level of risk exposure, such as using firewalls, keeping software up to date, and using intrusion detection and prevention systems.