Attacks against computers and the Internet are in the news every week. These primarily take the form of malicious code such as viruses and worms, or denial of service attacks. Less commonly reported are attacks which gain access to computers, either for the purpose of producing damage (such as defacing web sites or deleting data) or for the opportunities such access provides to the attacker, such as access to bank accounts or control systems of power stations. In a perspectives article in Science (Wulf and Jones 2009) the authors argue that computer systems are getting less secure, not more, and that traditional models of security based on perimeter defenses are not working.