# Legacy ICS Cybersecurity Assessment Using Hybrid Threat Modeling--An Oil and Gas Sector Case Study
## 2 Background and Related Work
### Industrial Control System (ICS)
The American Petroleum Institute (API) has developed Recommended Practices (RP), which are applicable to the security assessment of offshore oil and gas assets, both domestically and internationally. Table 1 illustrates a compilation of cyber security standards. The focus of this paper is on assessing legacy ICS systems, and the established frameworks serve as a critical foundation for the proposed assessment methodology. Understanding and incorporating best practices from these frameworks ensures the comprehensiveness and alignment of the proposed hybrid threat modeling approach with industry standards. The Open Web Application Security Project (OWASP) outlined the objectives of threat modeling. \"Threat modeling is a family of activities for improving security by identifying threats and then defining countermeasures to prevent, or mitigate the effects of, threats to the system\" [25]. A threat is a potential or actual undesirable event that may be malicious (such as a DoS attack) or incidental (failure of a storage device). Threat modeling is a planned activity that aims to identify and assess application threats and vulnerabilities. Safety is a critical concern for process industries. [PERSON] et al. [26] highlight that there is a need for an integrated approach to design the safety and security of CPS systems. Upon examining the physical behavior of the system, it became evident that not every attack could result in a system hazard; the development of a hazard could take some time, and it might be feasible to neutralize the attack. The authors presented attack scenarios on a testbed with real-world industrial controllers and communication protocols. An attack tree model is used to present the BPCS (basic process control system) runaway and data flow of the system. This paper explores the application of hybrid threat modeling techniques for cybersecurity assessments of legacy Industrial Control Systems (ICS) in the Oil and Gas sector. Several established threat modeling methodologies exist, each with its strengths and focus areas [27]. The selection of the most suitable approach depends on the specific project requirements and unique security concerns. The importance of threat modeling in this research is paramount. * Comprehensive Threat Identification: Threat modeling makes it easier to find a lot of different types of threats, which is important when you think about how many different people have a stake in cyber-physical systems like ICS [27]. * Proactive Security Design: By integrating threat modeling early in the assessment process, you can make proactive architectural decisions. This approach leverages security requirements to mitigate potential threats from the beginning, avoiding costly mitigation efforts later [27]. * Tailored Approach for Legacy Systems: Legacy ICS often lack modern security features, making them more vulnerable. A hybrid threat modeling approach, combining differ \begin{table}
\begin{tabular}{c l l} \hline \hline
**Standards** & **Originator** & **Title** \\ \hline \multirow{2}{*}{1} & NIST (National Institute of Standards and Technology) [21] & NIST Special Publications 800-30, 800-37, 800-82 \\ \hline \multirow{2}{*}{2} & ASTM (American Society for Testing \& Material) [22] & ASTM F3286-17, ASTM F3449-20 \\ \hline \multirow{3}{*}{3} & ISO/IEC (International Organization for & \\  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of standards related to cybersecurity. ent methodologies, can be particularly effective in uncovering these vulnerabilities within legacy systems. The evaluation of threat modeling methods (TMMs) against selected criteria, including strengths, weaknesses, adoptability, tailorability, perspective, applicability to CPS, and automation, led to the conclusion that no single method can comprehensively address these aspects. Thus, hybrid threat modeling provides a complete framework for assessing these sophisticated systems by combining multiple threat modeling techniques [21; 22; 23; 24].