A vulnerability is a weakness or defect that exists within technical, physical, or human systems that hackers can exploit in order to gain access to or control over systems within a network. Common vulnerabilities that exist within organizations include human error (the greatest of vulnerabilities on a global scale), web application injection vulnerabilities, and the oldest of vulnerabilities, the buffer overflow.

Now that we know what a vulnerability is, let's take a look at what is used by a hacker to take advantage of a security weakness in the next section.