The IT infrastructure consists of systems and the connections between them. Systems are what we typically call computers, servers, hosts, nodes, smartphones, devices etc. The design architecture of systems consists of an IT stack, which roughly contain the hardware layer at the bottom, operating systems in between and applications as the top layer. There might be mezzanine layers in between. Anyway, for a system to be secure, it is of course necessary that every layer is secure, and most developers of systems make a good effort to ensure adequate security. Unfortunately, the totality of the IT stack in a system is extremely complex, with millions of lines of code at each layer. For this reason, every system always has security vulnerabilities that will be exploited as soon as attackers can find them. There is a continuous race between attackers searching for and exploiting vulnerabilities, and vendors and organizations removing and mitigating the same vulnerabilities.