The application layer—layer seven in the OSI Reference Model—is what the lower layers are built for. The explosive growth of the Internet is made possible by the lower layers, but the applications that ride on top of these layers are the fuel that stokes the fire. There are thousands of Internet-enabled applications designed to make complex tasks easier and solve problems for everyone from consumers to governments to multinational corporations. A pervasive concern for all of these applications is security, and so far, judging from the rate of vulnerability announcements from sources like Bugtraq, the status quo is not working so well.
When it comes to breaking into systems, the application layer is where most of the action is. High-value targets such as interfaces to online banking and sensitive medical information exist at (or are accessible from) the application layer, and the threat environment today shows a trend toward attackers compromising systems for monetary gain. Along the way, the personal privacy of individuals is thrown by the wayside. If security requirements were treated with a higher priority at all phases of an application's life cycle—design, development, deployment, and maintenance—we would all be better off.
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