Computer security is difficult (maybe even impossible), but imagine for a moment that we've achieved it. Strong cryptography is where required; secure protocols are doing whatever needs to be done. The hardware is secure; the software is secure. Even the network is secure. It's a miracle.
Unfortunately, this still isn't enough. For this miraculous computer system to do anything useful, it is going to have to interact with users in some way, at some time, for some reason. And this interaction is the biggest security risk of them all. People often represent the weakest link in the security chain and are chronically responsible for the failure of security systems.
When I started doing cryptographic consulting for companies, I would tell prospective clients that I could secure their digital data more or less perfectly, but that securing the interaction between the data and the people would be a problem. Now I am more cynical. Now I tell prospective clients that the mathematics are impeccable, the computers are vincible, the networks are lousy, and the people are abysmal. I've learned a lot about the problems of securing computers and networks, but none of that really helps solve the people problem. Securing the interaction between people and just about anything is a big problem.
People don't understand computers. Computers are magical boxes that do things. People believe what computers tell them. People just want to get their jobs done.
People don't understand risks. ...