Several important systems like power grids, water supply systems, oil and gas production/distribution systems, mass security system quotes as critical infrastructure (CI) systems due to the catastrophic nature of damages that can result from their failure. The task of monitoring and controlling such systems is often entrusted to Supervisory Control and Data Acquisition (SCADA) systems.
SCADA systems are an attractive target for attackers, as they offer an avenue for launching attacks against high valued CI systems. A typical SCADA system may include several remote terminal units (RTU), one or more master terminal units (MTU), a variety of communication equipment and links, computers running human machine interface (HMI) software to enable more intuitive operator driven control when necessary. Hidden malicious/accidental functionality in any SCADA system component could be exploited by an attacker to launch attacks such as the above. Such hidden functionality could exist in (the logic programmed into) programmable logic controllers (PLC) in RTUs and MTUs, in any computer used for programming PLCs, or in any peripheral of the computer running the HMI software or the SCADA data logger, in the operating system of such computers, in the HMI software, or even, ironically, in a computer that runs the intrusion detection system (IDS) intended for protecting the SCADA system.
In 2010, a virus known as Stuxnet1 that had evaded detection for over a year  was identified. This virus targeted nuclear plants, and shut down centrifuges inside the plant by overwriting some set-points. In November 2011, the Illinois Statewide Terrorism and Intelligence Center reported2 a cyber-attack on a small, rural water utility outside Springfield, where attackers had gained remote access to pumps. In May 2003  , a Slammer worm exploiting3 an un-patched version of Microsoft SQL erased crucial SCADA system logs. “In March 2016, the U.S. Justice Department claimed that Iran had attacked U.S. infrastructure by infiltrating the industrial controls of a dam in Rye Brook, New York. The attackers compromised the dams command-and-control system in 2013 using a cellular modem”  . “In December 2015, a power company located in western Ukraine suffered a power outage that impacted a large area that included the regional capital of Ivano-Frankivsk. The cybercriminals had facilitated the outage by using BlackEnergy malware to exploit the macros in Microsoft Excel documents”  .
It is indeed for very good reasons that such threats have been recognized as “Advanced Persistent Threats”      . Due to the high value of targets, the possibility of sophisticated state sponsored attacks has to be considered. Sophisticated malicious functionality may be introduced even during the manufacturing process of various components that could ultimately end up in SCADA systems. In addition, we cannot afford to ignore the possibility that an attacker may have actually participated in the deployment of the SCADA system, or testing of the deployed system, and taken advantage of such an opportunity to inject hidden functionality in some component.
While it is important to take all possible practical steps to reduce the threat of hidden malicious functionality, we may never be able to eliminate such functionality in every component. Such functionality may be exploited to launch attacks while simultaneously reporting “all clear” messages to the stake-holders. It is of vital importance that we are at the minimum able to reliably detect such attacks, even if hidden malicious functionality is inevitable.
1.1. Active vs Passive Security Measures
The process of securing any system can be seen as consisting of three broad steps: 1) enumeration of desired assurances; 2) identification of reasonable assumptions; and 3) development of a process, viz., a security protocol, to translate the assumptions into the desired assurances. In other words, if we begin with good assumptions, and if the security protocol is correct, and if the agency responsible for executing the protocol is trustworthy, then the desired assurances are guaranteed.