What is zero-day exploit? - Definition from WhatIs.com. Sometimes, when vulnerabilities are found in software, the hacker who finds it is a good guy, and notifies the software company of the vulnerability in time to fix the problem. Microsoft has fixed such problems with the Windows OS every Tuesday for a long time. When your computer tells you that system updates are available, don't ignore them. Just do it. Other times, however, the hacker is a bad guy, and exploits a found vulnerability simultaneously with its discovery. Hence, zero days to fix it. I know that many of you find computer security arcane or boring. However, your life is in that little box. While you can never be 100% secure, that is no reason to leave your doors and windows open. Security lecture over for today.