A zero-day (also known as 0-day) vulnerability is a computer-software vulnerability that is unknown to those who would be interested in mitigating the vulnerability (including the vendor of the target software). Until the vulnerability is mitigated, hackers can exploit it to adversely affect computer programs, data, additional computers or a network. An exploit directed at a zero-day is called a zero-day exploit, or zero-day attack.

In the jargon of computer security, “Day Zero” is the day on which the interested party (presumably the vendor of the targeted system) learns of the vulnerability. Up until that day, the vulnerability is known as a zero-day vulnerability. Similarly, an exploitable bug that has been known for thirty days would be called a 30-day vulnerability. Once the vendor learns of the vulnerability, the vendor will usually create patches or advise workarounds to mitigate it.

The fewer the days since Day Zero, the higher the chance no fix or mitigation has been developed. Even after a fix is developed, the fewer the days since Day Zero, the higher is the probability that an attack against the afflicted software will be successful, because not every user of that software will have applied the fix. For zero-day exploits, the probability that a user has patched their bugs is zero, so the exploit should always succeed. Zero-day attacks are a severe threat.