What is a Zero Day Exploit?

Answer 1

A zero-day exploit is one that takes advantage of a security vulnerability on the same day that the vulnerability becomes generally known. Ordinarily, after someone detects that a software program contains a potential exposure to exploitation by a hacker, that person or company can notify the software company and sometimes the world at large so that action can be taken to repair the exposure or defend against its exploitation. Given time, the software company can repair and distribute a fix to users. Even if potential hackers also learn of the vulnerability, it may take them some time to exploit it; meanwhile, the fix can hopefully become available first.

With experience, however, hackers are becoming faster at exploiting a vulnerability and sometimes a hacker may be the first to discover the vulnerability. In these situations, the vulnerability and the exploit may become apparent on the same day. Since the vulnerability isn't known in advance, there is no way to guard against the exploit before it happens. Companies exposed to such exploits can, however, institute procedures for early detection of an exploit.