A zero-day vulnerability is a security flaw in software or hardware that is unknown to the vendor or developer and has not yet been patched. Because the vulnerability is undiscovered, there are no existing defenses or fixes, making systems particularly susceptible to attacks that exploit this weakness. When attackers exploit a zero-day vulnerability, it can lead to unauthorized access, data breaches, and other malicious activities. The term “zero-day” signifies that the developers have had zero days to address and mitigate the vulnerability, highlighting the urgency and risk associated with such security flaws.