## 1 Introduction  
Zero trust is a strategic approach to cybersecurity that secures an organization by eliminating implicit trust and continuously validating every stage of digital interaction [1]. There is no generic definition of zero-trust, but it is seen as a security model which considers all sensitive data are not necessarily protected by firewalls and as an access control approach, trust must be established every time removing any assumptions from past decisions [2]. Most of the business applications and resources nowadays reside outside of the traditional perimeter. There are various options for organizations to choose from depending on their business needs and infrastructure. This migration from on-premises to public cloud or hybrid cloud environments results in more challenging cybersecurity issues, most of which are related to user access. The Zero-trust security approach is about eliminating the implicit trust between users, applications, and infrastructures within or outside of an organization's network. It is a methodology that considers the entire ecosystem of controls rather than just focusing on the narrow technology [1]. In 2011, the German federal government announced Smart Manufacturing, also known as Industry 4.0 to be a key initiative in shaping the future of the manufacturing industry [3]. Smart manufacturing focuses on the end-to-end digitization of all physical assets and integration into digital ecosystems with value chain partners. Ecosystems form when a collaborative network of stakeholders, enabled by digital technology, come together in meaningful ways to meet shared objectives and solve shared challenges. The three key components of Smart Manufacturing are the Internet of Things (IoT), Cyber-Physical Systems (CPS), and Smart Factories. Digital ecosystems can only function efficiently if all parties involved can trust in the security of their data and communication, as well as the protection of their intellectual property. Protecting the company and ensuring digital trust requires significant investment and clear guidelines for data integrity and security [4]. Even today cybersecurity is considered one of the characteristics of smart manufacturing technology rather than it being an integral part of the design. The denial-of-service attack on Ukraine's power grid in 2015 [5] and the spear-phishing attack on Germany's steel factory in 2014 [6] are a couple of examples that reflect this misconception. The severity of attacks on smart manufacturing systems is highly catastrophic because the potential outcomes of these attacks not only damage the production line or quality of the products but can also range from economic breakdown to injury and loss of human life up to an extent of nationwide effects [7]. Designing a sustainable security model such as zero-trust for the future smart manufacturing systems will bring down the level of cyberattacks thus lowering the severity of the damage. Zero-trust model brings maximum security with some disadvantages like complexity, more manpower, slow down application performance, more cost and hamper productivity. These disadvantages and their mitigation are given in [8]. Zero-trust is not only securing traditional computing assets but has extended to the cloud and IoT spaces such as the one used for smart manufacturing alongside big data, cloud computing, AI, and machine learning [9]. The objective of this paper is to propose a zero-trust model for the smart manufacturing system which can secure the vulnerable communication channels between different manufacturing equipment such as the execution system and control system, the data that are sent to the cloud for analysis & intelligent control, and the admin access to these resources. It is important to understand that zero-trust is not just one security product that can be placed in the infrastructure and confirmed to be secured. It is rather a concept that involves multiple aspects of closing down the holes in an environment with the use of different security products.