The Siemens S7 protocol is commonly used in SCADA systems for communications between a Human Machine Interface (HMI) and the Programmable Logic Controllers (PLCs). This paper presents a model-based Intrusion Detection Systems (IDS) designed for S7 networks. The approach is based on the key observation that S7 traffic to and from a specific PLC is highly periodic; as a result, each HMI-PLC channel can be modeled using its own unique Deterministic Finite Automaton (DFA). The resulting DFA-based IDS is very sensitive and is able to flag anomalies such as a message appearing out of its position in the normal sequence or a message referring to a single unexpected bit. The intrusion detection approach was evaluated on traffic from two production systems. Despite its high sensitivity, the system had a very low false positive rate - over 99.82% of the traffic was identified as normal.
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Kleinmann, Amit and Wool, Avishai
"Accurate Modeling of the Siemens S7 SCADA Protocol for Intrusion Detection and Digital Forensics,"
Journal of Digital Forensics, Security and Law: Vol. 9
, Article 4.
Available at: http://commons.erau.edu/jdfsl/vol9/iss2/4