In the fast-growing internet applications, email becomes more and more important in communication. SMTP attacks and spam have become one of the most serious problems. Particularly, the SMTP attacks and spam varies on email, for example spoofing address, illegal characters, sending in bulk, too many SMTP commands and so on. A single security technique is not enough to protect the system from these attacks and spam. In this paper, we propose a SMTP Intrusion Prevention System (SIPS) which bases on the concept of Stateful Protocol Anomaly Detection and Flow-based Inspection. SIPS is implemented by a finite state machine to inspect all coming email flows. It is according to the media type of email flow and their characteristics. On the test of a real email environment, our approach can prevent attacks on SMTP attack (mail bomb) average about 95.4% and spam average about 91.1%.
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