‘The insecure computer’ outlines the main threats to computer security, and gives advice on how they can be countered. Malware is software that is introduced to a system illegally and which carries out malicious acts. It can be introduced via email, by failing to use firewalls and virus scanners, or by a trapdoor in poorly written software. Many computer crimes exist—some require a high degree of technical skill, others can be carried out by anyone. Defences against technological attacks include cryptography, the Secure Sockets Layer (SSL), passwords, and biometrics. Defences against non–technological attacks involve common sense approaches to general workplace security.