A timing approach to network-based anomaly detection for SCADA systems
Abstract: Supervisory Control and Data Acquisition (SCADA) systems control and monitor critical infrastructure in society, such as electricity transmission and distribution systems. Modern SCADA systems are increasingly adopting open architectures, protocols, and standards and being connected to the Internet to enable remote control. A boost in sophisticated attacks against SCADA systems makes SCADA security a pressing issue. An Intrusion Detection System (IDS) is a security countermeasure that monitors a network and tracks unauthenticated activities inside the network. Most commercial IDSs used in general IT systems are signature-based, by which an IDS compares the system behaviors with known attack patterns. Unfortunately, recent attacks against SCADA systems exploit zero-day vulnerabilities in SCADA devices which are undetectable by signature-based IDSs.This thesis aims to enhance SCADA system monitoring by anomaly detection that models normal behaviors and finds deviations from the model. With anomaly detection, zero-day attacks are possible to detect. We focus on modeling the timing attributes of SCADA traffic for two reasons: (1) the timing regularity fits the automation nature of SCADA systems, and (2) the timing information (i.e., arrival time) of a packet is captured and sent by a network driver where an IDS is located. Hence, it’s less prone to intentional manipulation by an attacker, compared to the payload of a packet.This thesis first categorises SCADA traffic into two groups, request-response and spontaneous traffic, and studies data collected in three different protocol formats (Modbus, Siemens S7, and IEC-60870-5-104). The request-response traffic is generated by a polling mechanism. For this type of traffic, we model the inter-arrival times for each command and response pair with a statistical approach. Results presented in this thesis show that request-response traffic exists in several SCADA traffic sets collected from systems with different sizes and settings. The proposed statistical approach for request-response traffic can detect attacks having subtle changes in timing, such as a single packet insertion and TCP prediction for two of the three SCADA protocols studied.The spontaneous traffic is generated by remote terminal units when they see significant changes in measurement values. For this type of traffic, we first use a pattern mining approach to find the timing characteristics of the data. Then, we model the suggested attributes with machine learning approaches and run it on traffic collected in a real power facility. We test our anomaly detection model with two types of attacks. One causes persistent anomalies and another only causes intermittent ones. Our anomaly detector exhibits a 100% detection rate with at most 0.5% false positive rate for the attacks with persistent anomalies. For the attacks with intermittent anomalies, we find our approach effective when (1) the anomalies last for a longer period (over 1 hour), or (2) the original traffic has relatively low volume.
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