A zero-day exploit, or zero-day attack, is a cyberattack that takes advantage of a previously unknown vulnerability in software or hardware, which the vendor or developer has not yet had an opportunity to patch or fix. These exploits are called “zero day” because they occur before or on the very first day the vulnerability becomes known, leaving zero days for defenders to protect against the attack. Zero-day exploits are highly dangerous because they can bypass traditional security measures and cause significant damage before being detected and mitigated.