Also known as: 0-day · zero day vulnerability · zero-day exploit
A zero-day is a software vulnerability that attackers know about and exploit before the vendor has released a patch, leaving defenders zero days to prepare a fix.
The term describes a timing relationship, not a type of bug. The same buffer overflow is a zero-day on Monday and an ordinary vulnerability on Friday if a patch shipped Wednesday. What makes it dangerous is that no official remediation exists at the moment of exploitation, so defenders are limited to mitigations: network isolation, disabling the affected feature, or virtual patching at a proxy layer.
Zero-days are frequently overstated in marketing material. Most successful intrusions use known vulnerabilities that went unpatched for months, not novel ones. A genuine zero-day is expensive to discover and burns permanently once used and detected, which is why they tend to be reserved for high-value targets rather than sprayed at the internet.
An attacker exploiting a flaw in an SSH library two weeks before the maintainers are even notified is using a zero-day. Once the patch ships and the CVE is published, subsequent exploitation of unpatched servers is no longer a zero-day — it is an n-day.
N-Day
An n-day is a vulnerability that has already been publicly disclosed and patched, but remains exploitable on systems that have not yet applied the update.
CVE
CVE (Common Vulnerabilities and Exposures) is a public catalogue that assigns a unique identifier, such as CVE-2026-31337, to each publicly disclosed software vulnerability so that everyone can refer to the same flaw unambiguously.
Exploit
An exploit is code or a technique that takes advantage of a specific vulnerability to cause unintended behaviour, such as executing attacker-supplied code, bypassing authentication, or crashing a service.
Responsible Disclosure
Responsible disclosure, also called coordinated vulnerability disclosure, is the practice of privately reporting a vulnerability to the affected vendor and allowing a defined period for a fix before publishing details.
Patch Tuesday
Patch Tuesday is the second Tuesday of each month, on which Microsoft and several other major vendors release their scheduled batch of security updates, giving administrators a predictable cadence for planning maintenance.