Zero day attack

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.
A study released by Symantec in early 2004 found that although the number of vulnerabilities discovered was about the same in 2003 as in 2002, the time between the vulnerability and exploits based on it had narrowed. According to the infoAnarchy wiki, "14-day" groups and "7-day" groups carry out an exploit within 14 or 7 days of a product's market release. Conducting a zero-day exploit establishes crackers as members of the elite, because they must have covert industry connections to gain the inside information needed to carry out the attack.

A zero-day (or zero-hour) attack or threat is a computer threat that tries to exploit unknown, undisclosed or patchfree computer application vulnerabilities. The term Zero Day is also used to describe unknown or Zero day viruses.
Zero-day exploits are released before the vendor patch is released to the public.
Zero-day exploits generally circulate through the ranks of attackers until finally being released on public forums. The term derives from the age of the exploit. A zero-day exploit is usually unknown to the public and to the product vendor.