N-day
Also called n-day vulnerability, known vulnerability, one-day
A security flaw attacked after its fix is public. The "n" is the number of days the fix has been available and not applied.
An n-day is a flaw that is already public and already has a fix. What makes it dangerous is the wait: every system that hasn't been patched is open to an attack that anyone can now study.
An attacker using an n-day doesn't need to discover anything. They take a flaw that has just been announced, build an attack for it, and scan the internet for systems that are still unpatched. Recent growth in attacks on software flaws appears to come mainly from this fast use of known flaws.
That is why the days after a fix is published matter so much, and why AI matters here. Building the attack used to take skilled people some time, and that time was the defender's margin. It has shrunk to days.
For example
A VPN maker publishes a fix on Monday; by Friday attackers are breaking into companies that haven't installed it.
Put it to use
Where you'll meet N-day in ChatGPT, Gemini and Claude, how to use it there, and the mix-up most people make.
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