In many cases, it s the problems lurking underneath the top layer that are the real killers. These less-noticed vulnerabilities are the ones that penetration testers and more worryingly, attackers use to dig deep into a target network. And often, they re not even proper vulnerabilities, but rather misconfigurations, services left exposed or inter-organizational trust relationships that can be used and abused by attackers to move from one machine to the next. The reality is that skilled attackers have time and human nature on their side.
Source: https://threatpost.com/dangers-second-order-vulnerabilities-091911/75670/