A zero-day vulnerability is a vulnerability in software that is unknown to its vendors (or users). Until the vulnerability is fixed (usually via a patch) it can be exploited by hackers for malicious purposes. Zero-day attacks that exploit the vulnerability therefore have a very high chance of success. The day that the vendor discovers the vulnerability is known as “day zero”. Vendors then rush to create a patch that will protect their software, and provide that patch to all users of it. The longer this takes, the more likely affected systems are to be attacked by hackers aware of the vulnerability.