A zero-day exploit refers to a security vulnerability in a software or system that is unknown to the vendor or developer. It is called "zero-day" because the developers have had zero days to fix the vulnerability before it is exploited by hackers.

Zero-day exploits are highly valuable to cybercriminals because they can be used to carry out attacks before the affected software or system has a chance to release a patch or update to fix the vulnerability. This gives the attackers a significant advantage, as they can target unsuspecting users who are unaware of the vulnerability.

Let's look at an example to understand this better. Suppose a popular web browser has a zero-day vulnerability that allows an attacker to execute arbitrary code on a victim's computer by simply visiting a malicious website. The attacker can exploit this vulnerability to install malware, steal sensitive information, or gain unauthorized access to the victim's system. Since the browser vendor is unaware of this vulnerability, they have not released a patch to fix it.

References:

• Symantec Threat Report
• Cybersecurity and Infrastructure Security Agency (CISA) - Zero-Day Vulnerability