A zero-day vulnerability, often referred to as a 'zero,' is a security flaw in software or hardware that is unknown to the vendor or developer. Because the manufacturer is unaware, no patch or fix exists, leaving the system exposed to attacks. Cybercriminals exploit zero-day vulnerabilities to gain unauthorized access, steal data, or spread malware before the issue is discovered and resolved. Zero-day attacks are particularly dangerous because they can go undetected for some time, making them highly sought after in the cybercriminal world.