Cyber-Physical Systems Security—A Survey <s> VIII. SECURITY CHALLENGES <s> The purpose of this document is to provide guidance for securing industrial control systems (ICS), including supervisory control and data acquisition (SCADA) systems, distributed control systems (DCS), and other systems performing control functions. The document provides an overview of ICS and typical system topologies, identifies typical threats and vulnerabilities to these systems, and provides recommended security countermeasures to mitigate the associated risks. Because there are many different types of ICS with varying levels of potential risk and impact, the document provides a list of many different methods and techniques for securing ICS. The document should not be used purely as a checklist to secure a specific system. Readers are encouraged to perform a risk-based assessment on their systems and to tailor the recommended guidelines and solutions to meet their specific security, business and operational requirements. The scope of this document includes ICS that are typically used in the electric, water and wastewater, oil and natural gas, chemical, pharmaceutical, pulp and paper, food and beverage, and discrete manufacturing (automotive, aerospace, and durable goods) industries. <s> BIB001 </s> Cyber-Physical Systems Security—A Survey <s> VIII. SECURITY CHALLENGES <s> In the last years there has been an increasing interest in the security of process control and SCADA systems. Furthermore, recent computer attacks such as the Stuxnet worm, have shown there are parties with the motivation and resources to effectively attack control systems. While previous work has proposed new security mechanisms for control systems, few of them have explored new and fundamentally different research problems for securing control systems when compared to securing traditional information technology (IT) systems. In particular, the sophistication of new malware attacking control systems--malware including zero-days attacks, rootkits created for control systems, and software signed by trusted certificate authorities--has shown that it is very difficult to prevent and detect these attacks based solely on IT system information. In this paper we show how, by incorporating knowledge of the physical system under control, we are able to detect computer attacks that change the behavior of the targeted control system. By using knowledge of the physical system we are able to focus on the final objective of the attack, and not on the particular mechanisms of how vulnerabilities are exploited, and how the attack is hidden. We analyze the security and safety of our mechanisms by exploring the effects of stealthy attacks, and by ensuring that automatic attack-response mechanisms will not drive the system to an unsafe state. A secondary goal of this paper is to initiate the discussion between control and security practitioners--two areas that have had little interaction in the past. We believe that control engineers can leverage security engineering to design--based on a combination of their best practices--control algorithms that go beyond safety and fault tolerance, and include considerations to survive targeted attacks. <s> BIB002 </s> Cyber-Physical Systems Security—A Survey <s> VIII. SECURITY CHALLENGES <s> It is often appealing to assume that existing solutions can be directly applied to emerging engineering domains. Unfortunately, careful investigation of the unique challenges presented by new domains exposes its idiosyncrasies, thus often requiring new approaches and solutions. In this paper, we argue that the “smart” grid, replacing its incredibly successful and reliable predecessor, poses a series of new security challenges, among others, that require novel approaches to the field of cyber security. We will call this new field cyber-physical security. The tight coupling between information and communication technologies and physical systems introduces new security concerns, requiring a rethinking of the commonly used objectives and methods. Existing security approaches are either inapplicable, not viable, insufficiently scalable, incompatible, or simply inadequate to address the challenges posed by highly complex environments such as the smart grid. A concerted effort by the entire industry, the research community, and the policy makers is required to achieve the vision of a secure smart grid infrastructure. <s> BIB003
Although the terminology of challenges, threats, and vulnerabilities are sometimes used interchangeably, we use each terminology to shed light on a different aspect of CPS security. Challenges are open problems that are still unsolved and we try to stimulate research efforts toward such problems. On the other hand, we follow the definition of threats, vulnerabilities, attacks, and controls in the general security literature  . Roughly, vulnerabilities are internal (security) weaknesses of a system that may be exploited by adversaries, while threats are external circumstances that are potentially harmful. In this section, we introduce CPS challenges based on our observations from surveying the literature and classify them into general and application-specific challenges. A. General CPS Security Challenges 1) Security by Design: Security is not taken into consideration in the design of most CPS as a result of their isolation in physically secured environments without connectivity to other networks, the Internet for instance. Hence, physical security has been almost the main security measure BIB001 . 2) Cyber-Physical Security: The security mindset of CPS designers needs to change so that they consider both cyber and physical aspects. This way potential cyber-attacks with physical consequences will be better predicted and thus mitigated  . Neuman  suggested that when the fundamental differences between cyber and physical aspects are not considered, cyber-physical solutions are usually ignored, and the focus becomes cyber-only solutions. This urges the need for considering both cyber-and physical-aspects. Mo et al. BIB003 described a new field called "cyber-physical security." The authors' aim to help by their proposal in developing novel solutions for the CPS security issues, especially in the context of smart grids. Furthermore, the survivability of the systems under attack is very crucial in CPS. A set of CPS security challenges, such as survivability, are discussed in  . 3) Real-Timeliness Nature: The real-time requirement is a requirement whose absence affects the security posture BIB002 ,  . During a security attack, real-time decisions in CPS are crucial for systems' survivability. Therefore, consideration of the interactions between physical-and cyberaspects in any CPS security design gives the full picture of the system, which assists in designing better risk-assessment, attack-detection, and attack-resilient solutions BIB002 . In addition, cryptographic mechanisms could cause delays that could affect some real-time deadlines. Therefore, lightweight and hardware-based mechanisms should be considered.