# Cybersecurity Requirements for Industrial Machine Control Systems
## 4 Relationship Between Safety Categories and PLs and Security Levels  
As mentioned in the Introduction, one of the potential targets of cyber criminals can become the machine's safety systems. A real danger then arises for an operator and maintenance service if they undertake work in machine zones that pose a risk, which should be protected by the control system (e.g., ensuring low speeds to avoid danger; \begin{table}
\begin{tabular}{c c} \hline \hline
**Category** & **Features** \\ \hline \multirow{3}{*}{B} & Use of elements that comply with standards and basic safety principles, withstanding expected exposures. Occurrence of failure may result in loss of safety function (single-channel system) \\ \hline \multirow{3}{*}{1} & As in B, well-tried components and well-tried safety principles are used. The occurrence of a failure may cause a loss of safety function, but the probability of this is lower than in category B (single-channel system) \\ \hline \multirow{3}{*}{2} & As in B, well-tried safety principles are used. The safety function is checked by the control system. The occurrence of a fault may cause the loss of the safety function between checks (single-channel system) \\ \hline \multirow{3}{*}{3} & As in B, well-tried safety principles are applied. The two-channel structure is tolerant to a single fault. Low-to-medium diagnostic coverage of 60 to 99% \\ \hline \multirow{3}{*}{4} & As in B, well-tried safety principles are applied. The two-channel structure is tolerant of single failure or cumulative failures. High diagnostic coverage \(\geq\)99\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of safety categories based on ISO 13849-1. Figure 4: Architecture of redundant systems for category 3 (own elaboration). Figure 5: Architecture of redundant systems with the highest diagnostic coverage for category 4 (own elaboration). stopping when a light curtain zone is violated; controlling process parameters such as temperature or liquid level). It is therefore important to link safety-related machine control system requirements with cybersecurity requirements. However, the stability and cybersecurity of the entire hierarchical infrastructure, which includes interconnected hardware, software, and network components, also plays a crucial role. This hierarchy often comprises multiple layers, from local control systems to enterprise-level networks and cloud-based solutions. Each layer introduces specific vulnerabilities and requires tailored security measures. For example, while the network layer may be prone to denial-of-service attacks, the application layer could be susceptible to data breaches. Analyzing the interdependencies and failure propagation within this hierarchy is essential to ensure not only the safety of individual machines but also the resilience of the entire infrastructure. Future studies should incorporate multi-layered risk assessment frameworks, such as those outlined in IEC 62443 group of standards and ISO/IEC 27001 [21], to address these challenges. The technical specification IEC TS 62443-1-1 [22] Industrial communication networks--Network and system security--Part 1-1: Terminology, concepts and models recommends at least three SLs (Security Levels 1, 2, 3)--Table 3--while IEC 62443-4-2:2019 [23] Security for industrial automation and control systems--Part 4-2: Technical security requirements for IACS components (industrial automation and control system(s)) gives four SLs (Security Levels 1, 2, 3, 4). Informative Annex A of IEC 62443-3-2 [24] states that IEC 62443-4-2 [23] defines SLs at four different levels (1, 2, 3, and 4). The higher the SL, the greater the level of security. SL 0 indicates that there are no requirements in this area. * SL 1--protection against casual or coincidental violation. * SL 2--protection against intentional violation using simple means with low resources, generic skills, and low motivation. * SL 3--protection against intentional violation using sophisticated means with moderate resources, IACS-specific skills, and moderate motivation.