# A Proposal for a Zero-Trust-Based Multi-Level Security Model and Its Security Controls
## 4 Proposed Security Controls for the Zero-Trust-Based MLS Model and Its Robustness  
### Security Controls  
Security controls refer to policies, procedures, technologies, and other measures designed to minimise security risks to organisational assets and enhance the overall security level [41]. Security controls are generally categorised into the following three types based on their nature: administrative, technical, and physical controls [42]. These are described as follows:  
* Administrative controls: Security controls that manage security through policies, procedures, guidelines, and employee training. * Technical controls: Security controls that utilise technical means, such as security solutions or equipment, to protect the organisation's information assets. * Physical controls: Security controls aimed at protecting the organisation's facilities or infrastructure from physical threats. The organisation's security manager selects the necessary security control measures based on the organisational environment and information assets, ensuring their appropriate application. This approach helps to minimise damage in the event of incidents, such as security breaches, and enables the establishment of a more secure organisational infrastructure [43]. In this section, the security controls required for the model proposed in Section 4 are provided. Security controls to be applied to the model's key components are categorised according to each data classification. Referring to NIST SP 800-53 [18], the security controls were selected and differentially categorised based on the level of data sensitivity. First, the security controls that should be applied across the proposed model as a whole were involved in general. Additionally, specific controls required for key components of the model, including PDP, PEP, Resource, and Subject, were identified accordingly. Figure 5 summarises the security controls for the ZT-based MLS model. These security controls consist of general security controls that can be applied across all components of the proposed model and specific security controls for each of the four components. Furthermore, the required security controls vary according to the sensitivity of the organisational data, which are classified into the following three levels: C, S, and O. Tables 8-12 outline the security controls that should be applied for the key components of the model proposed in Section 4 to implement an MLS framework. The leftmost column specifies the overarching security controls, with newly assigned numbers based on the security controls defined in NIST SP 800-53. The central column details the specific security controls corresponding to the overarching security controls, categorised by the level of security required for each classification. The labels in parentheses, such as (AC-17), refer to those described in NIST SP 800-53. For C-level data and systems, the highest level of security controls, equivalent to those of the traditional network segregation framework, was  
Figure 5: Summary of security controls for ZT-based MLS model. applied. For S-level data and systems, high-level security controls satisfying the highest maturity level (Optimal) defined in CISA's Zero Trust maturity model [39] were applied. Finally, for O-level data and systems, security controls were applied to ensure availability and integrity while considering data usability aspects.