What characterizes a zero-day vulnerability?

A zero-day vulnerability is characterized by being a flaw that is exploited before a fix is available. This means that the vulnerability is unknown to the vendor and often to the public, allowing attackers to exploit it without any immediate defenses or patches in place. The term "zero-day" indicates that the flaw has zero days’ worth of protection against attacks, as it has just been discovered or is still unknown at the time of exploitation.

When a vulnerability is labeled as zero-day, it signifies a critical security risk for organizations because they have no time to prepare for or mitigate the threat. As soon as an attacker discovers the vulnerability, they can leverage it to compromise the affected systems or software until a patch is developed and deployed by the vendor. This context underscores the urgency and severity of zero-day vulnerabilities in cybersecurity, highlighting why they are a top concern for IT security professionals.