In short, a zero-day vulnerability is a security flaw in software, hardware, or firmware that the original developer or vendor is unaware of before it is exploited. The term "zero-day" indicates that the software developer or vendor has had no time (zero days) to patch or provide a workaround to mitigate the vulnerability. This type of vulnerability is considered one of the most dangerous, as there is no available patch or workaround to address the flaw. Consequently, the vendor requires time to investigate the flaw once it becomes known, giving malicious actors the opportunity to exploit it.
Moreover, zero-days are highly sought after by malicious actors and intelligence agencies, as they provide an advantage over their adversaries. They allow for the exploitation of remote systems without detection and can be leveraged to carry out targeted attacks, gain unauthorized access, execute malicious code, exfiltrate data, and insert backdoors.
Furthermore, zero-days are challenging to detect as they are "unknown." For instance, an Incident Response Team may handle a security breach, discover and close a backdoor. However, if they are unaware of the root cause of the breach, they are likely to experience repeated breaches.
