Practical software security metrics and measurements are essential to the development of secure software [18]. In this paper, we propose to use a software system's attack surface measurement as an indicator of the system's security; the larger the attack surface, the more insecure the system. We formalize the notion of a system's attack surface using an I/O automata model of the system [15] and define a quantitative measure of the attack surface in terms of three kinds of used in attacks on the system: methods, channels, and data. We demonstrate the feasibility of our approach by measuring the attack surfaces of two open source FTP daemons and two IMAP servers. Software developers can use our attack surface measurement method in the software development process and software consumers can use the method in their decision making process.

21 pages

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