A zero-day vulnerability is a type of computer security bug that attackers use to take control of systems, usually without the owners' knowledge or consent. These attacks are very difficult to defend against because they exploit unknown vulnerabilities in software and hardware.
Zero-day exploits are valuable as they have a high market value, providing a greater incentive for hackers to develop and sell them than non-zero-day exploits. Zero-day exploits are generally used in targeted attacks such as advanced persistent threats (APTs).
Zero-day vulnerabilities are often exploited by governments as part of their cyberweapons programs, with the goal being to gain access to foreign networks or infrastructure without being detected by anti-virus software or antivirus researchers.