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    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Cybersecurity in Distributed Industrial Digital Twins: Threats, Defenses, and Key Takeaways</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Sani M. Abdullahi</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Ashkan Zare</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Sanja Lazarova-Molnar</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Institute AIFB, Karlsruhe Institute of Technology</institution>
          ,
          <addr-line>Kaiserstr. 89, Karlsruhe, 76133</addr-line>
          ,
          <country country="DE">Germany</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Maersk Mc-Kinney Moller Institute, University of Southern Denmark</institution>
          ,
          <addr-line>Campusvej 55, Odense 5230</addr-line>
          ,
          <country country="DK">Denmark</country>
        </aff>
      </contrib-group>
      <abstract>
        <p>Distributed Digital Twins are designed to enhance the intelligence, predictability, and optimization of industrial assets by actively engaging, synchronizing, and collaborating with their physical counterparts, i.e., the systems they model, in near real time. This interoperability allows for seamless connections between real systems and their virtual counterparts, thereby facilitating the flow of data while aggregating vital information for comprehensive insights across large entities. However, the constant exchange of data and dependency on the information technology and operations technology process integrations in these complex distributed systems give rise to various cyber-security challenges. These include threats to data, unauthorized accesses, as well as threats to the integrity and reliability of the digital tools and the services they offer, among others. In this paper, we discuss the relevant cyber-threats within distributed Digital Twins ecosystems, which we then analyze while outlining different strategies to mitigate such threats. As a result, we present key takeaways toward a secure and reliable Digital Twin platform. Finally, different challenges are raised to highlight the status quo on the security of Digital Twins and areas for improvement.</p>
      </abstract>
      <kwd-group>
        <kwd>1 Distributed ditigal twins</kwd>
        <kwd>cybersecurity</kwd>
        <kwd>cyberthreats</kwd>
        <kwd>defense mechanisms</kwd>
        <kwd>manufacturing</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        According to the initial goals of Industry 4.0, the integration of new information technology (IT)
and operation technology (OT) paradigms into manufacturing, healthcare, automation, production, and
logistics is currently underway [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. This has led to the inception of some advanced technologies such
as AI, big data, IIoT, fog computing, edge computing, etc. However, one of the most notable amongst
such technologies within Industry 4.0 is the Digital Twin (DT). The primary objective of a DT is to
predict impacts of errors, variations, and significant deviations that could affect the inherent behavior
of a system by transforming physical assets into digital data representations via specification-based
methods [
        <xref ref-type="bibr" rid="ref2 ref3 ref4">2-4</xref>
        ], computational models [
        <xref ref-type="bibr" rid="ref5 ref6">5, 6</xref>
        ], and application programming interfaces (APIs) [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ]. These
methods operate on servers, virtual machines, virtual networks, and containers. Consequently, these
servers establish connections with physical elements so that they can engage in interaction with
realworld assets [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. As a result, Grieves [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] identifies a DT with three primary spaces, including physical,
digital, and communication, while Kritsinger et al. [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ] identify three different types of mirroring
systems, including digital model, digital shadow, and Digital Twin. All mirroring systems depict the
in-depth characterization of DTs through their integration with algorithms, networked systems, and
different technologies to make decisions that enable autonomous actions on physical entities.
      </p>
      <p>
        Up to this point, the practical usefulness of DTs has been demonstrated in several situations, such
as industry [
        <xref ref-type="bibr" rid="ref11 ref12">11, 12</xref>
        ], military field [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ], smart cities [14], and disaster management [15]. Such usefulness
of DTs in different application domains has attracted a plethora of private and public economic sectors,
as well as researchers and experts. However, insufficient investigation into cybersecurity concerns
persists, posing a dilemma for two primary reasons: i) Due to the involvement of DTs in automation
processes [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ], they are regarded as critical systems; and ii) DTs harbor private intellectual property that
serves as a digital replica of the physical real-world systems [16]. An attacker seeking to compromise
an industry's model will end up tarnishing its prestige or inflicting irreversible harm, especially with
regard to critical infrastructure. To seize control of the fundamental infrastructure and its physical
assets, an adversary may also cause damage to the DT from the digital space, as is evident when
contemplating a general DT scenario [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. This is a result of the DT paradigm's interconnectedness and
reliance on algorithms, cloud, and communication systems from the IT realm and cyber physical
technologies from the OT realm, which ends up linking the two and exposing it to more cyberthreats.
Moreover, this obviously suggests that the attack surface within the DT ecosystem is extremely diverse.
As such, the importance of cybersecurity for industrial Digital Twin cannot be overlooked.
      </p>
      <p>To shed more light on possible solutions regarding the plethora of cyberthreats that can inflict
irreparable damage to DTs, in this paper, we analyze the different cyberthreats within the DT ecosystem
while outlining some recent mitigation strategies. Moreover, we itemize key takeaways from our
exploration to ensure a secure and hitch-free deployment of DTs. In light of this, the remaining sections
of the paper are organized as follows: Section 2 provides background on the use of DT in industrial
domains as well as the role of cybersecurity in such DT in industrial environments. Section 3 discusses
the potential cybersecurity threats in industrial DT while delineating the recent strategies put forward
to mitigate such threats. Section 4 provides key takeaways from this overview to pave the way for future
research. In addition, different challenges will be given in Section 5, while Section 6 finally draws the
conclusion.</p>
    </sec>
    <sec id="sec-2">
      <title>2. Background</title>
    </sec>
    <sec id="sec-3">
      <title>2.1. Distributed DTs and their use in industry</title>
      <p>
        Since the Second World War, when computer simulation was first used [17], it has developed and
become a powerful tool and a key enabler in productivity, safety, optimization, and decision making in
industry today. In the early 2000s, Grieves introduced the concept of digital DT, a more complex near
real-time simulation model, representing the actual physical system in the virtual world throughout its
lifecycle [18]. From the time when Digital Twin was first defined, it has taken different definitions
depending on the application and the specific use cases. The three essential characteristics in these
definitions are: the physical object in the real world, the virtual representation of the object, and the
bidirectional exchange of data between the two objects [19]. In general, a Digital Twin can be defined
as “a virtual representation of a physical system (and its associated environment and processes) that is
updated through the exchange of information between the physical and virtual systems” [20]. Moreover,
depending on the automation level of data exchange between the two systems, DTs can also be further
categorized as digital models (manual data flow), digital shadows (one-way automatic data flow), and
Digital Twins (complete automatic data flow) [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ].
      </p>
      <p>Technological advancements, especially in Internet of Things (IoT) and sensor technologies, have
aided in realization of DT with automatic data flow as evident by the rapid growth in research and
application of DTs during the past decade [21]. Today, many industries benefit from the implementation
of DTs such as: aerospace, energy, agriculture, and manufacturing [22, 23]. The aerospace field, driven
by NASA, was the first industry to use DTs in performance optimization [24]. Energy and healthcare
industries have also utilized the capabilities of DTs for maintenance monitoring [25], and productivity
improvements [26]. More significantly, the vast applications of DTs in manufacturing industry show
DTs‘ pivotal role as a key catalyst in moving the industry towards smart manufacturing and Industry
4.0 [27]. DTs facilitate manufacturing testing and validation [28], as well as automation integration [29]
leading to production optimization and reliability improvements. Figure 1 illustrates DT’s concept.
2.2.</p>
    </sec>
    <sec id="sec-4">
      <title>Cybersecurity in industry</title>
      <p>As increasing number of industries transition to the digital environment and integrate their
operations online, cybersecurity is expanding into a more comprehensive and broader discipline. This
indicates that, apart from cybersecurity representing an industry on its own, there are cybersecurity
requirements in virtually every industrial sector. The notion of security has evolved beyond the typical
hacker symbolism of constantly crafting malicious code and breaking into systems; cybersecurity as a
discipline now incorporates a vast range of interdependent skills that collaborate harmoniously. These
skills include information security, network security, digital forensics, cloud security, critical
infrastructure security, and application security, among others.</p>
      <p>
        With the growing interconnectedness of IT-OT paradigms in industry, the potential vulnerabilities to
cyberattacks also expands. It is worth noting that such IT-OT integration is exacerbated by the
transformation of traditional industrial manufacturing systems into contemporary industrial
cyberphysical systems, which consequently necessitates the adoption of new technologies such as Digital
Twins that are complex and distributed in nature. Reports on industrial risk evaluations indicate that
this transformation has resulted in the emergence of several novel threats [30], which has further been
proven by the recent analysis on the distribution of cyberattacks across industries worldwide [31].
According to the study, manufacturing sector experienced the greatest proportion of cyberattacks
among the world's leading industries in 2022, with cyberattacks targeting manufacturing industries
comprising approximately 25% of the overall attacks during the year under study, with a total global
market size expected to project at an annual compound growth rate of 12.3% from 2023 to 2030 [32].
To establish an intelligent manufacturing setting based on automated decision making and problem
solving according to Industry 4.0 requirements, it is necessary to use innovative technologies that can
provide spontaneous connections between all industrial tools and devices within a manufacturing
ecosystem and the web, such as distributed DT, AI, IIoT, CPS, big data, etc. Moreover, Industry 4.0
promotes the use of these technologies to facilitate decentralized communication between systems
rather than just depending on conventional cloud servers or other centralized frameworks [
        <xref ref-type="bibr" rid="ref1">1, 33</xref>
        ].
Incorporating this diverse range of technologies within industrial cyberspace necessitates the
prioritization of cybersecurity issues in their design approach. While Industry 4.0 has proven to enhance
manufacturing efficacy and productivity, intrusions and breaches through cyberattacks can have severe
consequences for industrial operations, thus leading to the loss of sensitive information and credibility
[34]. Figure 2 depicts the interoperability between different novel technologies within the I4.0
ecosystem that are vulnerable to cyberthreats.
      </p>
      <p>Recent works on cybersecurity vulnerabilities in various industries and their possible defense
mechanisms include the works of Aoun et al. [35], where the authors explore the future prospects of
blockchain technology as a means of strengthening I4.0 industrialization. With the addition of novel
functionalities, they determine which domains of blockchain technology can enhance the
implementation and efficiency of I4.0, especially in terms of cybersecurity. In [36], the authors elucidate
the need for adopting a dynamic cybersecurity approach in response to the demands of Industry 4.0.
They analyze the limitations of present techniques and highlight the growing significance of integrating
novel approaches.</p>
      <p>Smart manufacturing</p>
      <p>AI, cobots, IIoT, DT, Augmented reality, etc
IT</p>
      <p>Convergence
• Data analytics
• Network connectivity
• Data processing
• Computing devices
• Storage devices
• Event management</p>
      <p>OT</p>
      <p>Laghari et al. [37] present a security system that utilizes digital signatures to provide authentication,
integrity, and defense against cyber breaches. The method is evaluated based on industrial
Semiconductor Equipment Communication Standard/Generic Equipment Model (SECS/GEM) design
and implementation protocols [36] against cyberattacks. The findings suggest that SECS/GEM
successfully blocked unauthorized organizations from initiating connections with legitimate industrial
devices while also ensuring the integrity of communications by rejecting fake signals. Likewise, attacks
such as replay, Denial-of-Service, and False-Data-Injection are mitigated by the SECS/GEM. In [38],
Ghimire et al. adopt a federated learning framework for cybersecurity within the IoT, with a main
emphasis on security. They also provide different advances to tackle performance concerns in terms of
accuracy, resource constraints, and latency. Latino et al. [39] proposed a cybersecurity reference
framework for the food and beverage industry. They compare the progress made in cybersecurity within
the food and beverage sector with suggested advancements in the I4.0 framework. Given the fact that
I4.0 has diverse implementations in the food and beverage supply chain, it also establishes unique
circumstances that necessitate ad hoc cybersecurity solutions. As such, the authors proposed future
directions to mitigate these challenges. Peña Zarzuelo [40] highlights some crucial cybersecurity
challenges in the ports and maritime industry. The author stressed the recent smart transformation of
the port sector (smart port) due to the increasing adoption of I4.0 and emphasized the importance of
ensuring its security as a critical infrastructure. Other proposed frameworks for different cybersecurity
application domains include space [41], automated vehicles [42, 43], process control and operations
[44], FinTech [45], energy [46], construction [47], railway [48], smart electric vehicle charging [49],
unmanned aerial systems [50], bioprinting [51], SMEs [52], medical devices, [53] etc.</p>
      <p>In [54], the authors combined deep learning and blockchain to secure critical health care data in
CPS. The authors present a secure pattern-proof malware validation technique that was developed
specifically for ICPS using blockchain and reinforcement learning techniques on the LSTM deep
learning model. The method is able to improve system performance, mitigate cyber threats, and detect
known and unknown attacks. Other applications of blockchain-based techniques for industrial
cybersecurity include the works on blockchain-secured smart manufacturing [55], blockchain in
internet of things [56], blockchain in industrial context [57], blockchain-enabled smart operations [58],
blockchain and DT empowered self-healing [59], and blockchain-based data-driven control system
[60].</p>
    </sec>
    <sec id="sec-5">
      <title>3. Cybersecurity in distributed industrial DTs</title>
      <p>The advancements brought by DTs in industry are undeniably huge. However, the interactions,
interconnectedness, and incorporation of DTs with other novel technologies, such as AI, big data, IIoT,
CPS, and enhanced visualization, present a variety of cybersecurity challenges. Moreover, given its
huge dependency on data, DTs are also vulnerable to multifaceted threats due to lack of data protection,
confidentiality, integrity, availability, and privacy.</p>
      <p>Hence, as we progressively embrace and incorporate distributed DTs in various industrial sectors
and everyday routines, it is crucial to fully understand and investigate their cybersecurity impacts. By
acquiring this insight, we will be able to fully utilize the advantages of DTs while averting the related
cybersecurity vulnerabilities. The main objective of this section is to examine the potential
cybersecurity threats in distributed DTs and outline methods to reduce them, with a specific emphasis
on the manufacturing sector.
3.1.</p>
    </sec>
    <sec id="sec-6">
      <title>Potential cyberthreats in distributed DTs</title>
      <p>Distributed DT ecosystems may, in fact, be more susceptible to cybersecurity breaches and threats
than conventional systems. This is due to the fact that platforms containing distributed DTs generally
exhibit different attributes, which include.
• The infrastructure is characterized by extensive network connectivity and redistribution.
• Distributed DT allows for remote management and exchange of vital information.
• Distributed DT connects to cloud, edge, and fog devices located at the perimeter that require a
highly secure system setting.
• Distributed DT platforms make use of open standards of operation.</p>
      <p>
        Additionally, security is a critical concern that needs to be taken into account within the distributed
DT platform. An important factor contributing to this is the expansion of the DTs to encompass a variety
of applications, such as data analytics, predictive maintenance, monitoring, etc., a significant number
of which are considered crucial. Moreover, each of these services is highly dependent on software
components including algorithms, models, and applications, which are frequently vulnerable to various
threats stemming from defects. Also, these services depend on numerous interfaces, interconnections,
and frameworks [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. The intricate nature of these nuances has the potential to introduce inaccuracies
into the DT, which could subsequently impact the operation of the underlying system via erroneous
conclusions.
      </p>
      <p>
        Furthermore, under the assumption that DTs can be modified to function in critical settings such as
the manufacturing industry, it becomes obligatory to provide additional protection for industrial systems
in conjunction with their DTs. Nevertheless, this requirement also prompts an investigation into whether
the integration of security protocols into the DT could subsequently introduce additional hardware and
software intricacies that could impact the functioning of the DT. One potential drawback is that the
integration of security measures may impede critical functions in DTs that encounter substantial
challenges in generating and analyzing models and data without having the choice to transfer resources
to more powerful platforms [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. Then, operational efficiency must take precedence in simulations
conducted on systems where security is a prerequisite but not a primary concern, provided that it does
not significantly impede tasks related to modeling and simulation [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ].
      </p>
      <p>
        Clearly, inadequate security setup within the distributed DT paradigm can also present a crucial
threat. Given that DTs are regarded as reflections of the real-physical world, they contain duplicate
confidential data from the whole IT-OT paradigm [61]. This data might comprise sensitive information
such as the operational procedures of OT components, the functional attributes of confidential
processes, the particulars of the operational setting and its connections, or the crucial security
credentials required to access vital resources. Consequently, DTs also encompass vital information,
enabling malicious actors to extract and generate a mapping of the complete system or a specific
component thereof, in addition to deriving confidential data or identifying patterns through the analysis
of databases, system structures, and resources [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. Furthermore, deliberate manipulation of digital assets
can have catastrophic consequences for their real-physical equivalents, as they are capable of automatic
decision-making. As a result, DTs must be regarded as critical platforms that need careful consideration
with respect to every security requirement, including confidentiality, integrity, availability, and privacy
of data and resources.
      </p>
      <p>
        In line with this, it is imperative that every security evaluation of DTs considers the four featured
functional layers [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ] in terms of their enabling technologies and the conceptualization provided in [
        <xref ref-type="bibr" rid="ref7">7,
62</xref>
        ]. This is primarily due to the fact that DTs predominantly depend on data processing and digital
assets, which necessitates the utilization of the four layers. Figure 3 depicts the range of cyberattacks
that the DT platform can be susceptible to based on the four functional layers. It can be seen that the
attack surface is extremely vast. Distributed DTs can be vulnerable to adversaries that exploit the
physical attack surface, which includes Layer 1 and Layers 2-4. However, physical assets might also
be vulnerable whenever DTs are targeted, specifically from Layers 4–2 and down to Layer 1. In this
scenario, both internal and external attackers might enhance their understanding and methods of attack
by gathering vital information directly from the DT source [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. This consequently leads to a devastating
attack sequence that could be detrimental to not just the DT but the entire industrial chain. All attacks
covered in Figure 3 emanate due to a variety of threats that clearly do not respect the security
requirement on confidentiality, integrity, availability, and privacy of data within the DT paradigm.
Below, we itemize such threats and their impact on the DT.
      </p>
      <p>Cyberthreats in the DT platform
Layer 1</p>
      <p>Layer 2 &amp; 3</p>
      <p>Layer 4
Cyber Physical Systems/IIoT</p>
      <p>Computing Infrastructure</p>
      <p>Visual Systems</p>
      <p>Computing Techniques</p>
      <p>HMIs</p>
    </sec>
    <sec id="sec-7">
      <title>3.1.1. Threats to data integrity</title>
      <p>The vulnerability of data poses a greater risk to the reliability and accuracy of DT. This can
compromise the integrity of the data it gets from the real physical system. Any tampering or distortion
of this data can result in inaccurate modeling and analysis, which in turn leads to flawed conclusions.
Data integrity-focused cyberattacks, such as tampering, sniffing, and manipulation through different
software attacks, present a substantial risk to distributed DTs [63] as they cover a large number of attack
surfaces that can be initiated at any of the four layers in Figure 3.</p>
    </sec>
    <sec id="sec-8">
      <title>3.1.2. Threats to data confidentiality</title>
      <p>Data confidentiality pertains to securing data from illicit access, disclosure, or unauthorized
intrusion, exposure, and misappropriation. An unsecure DT ecosystem will certainly fall short of
achieving the required confidentiality due to the huge attack surface. The significance of confidentiality
in distributed DTs cannot be overstated, particularly in ensuring the preservation of information and
data privacy. In essence, when establishing connections between DTs, particular confidentiality threats
from Layers 1 to 4 that the current security measures might not be able to adequately handle can arise
[64]. Therefore, a lot of considerations, such as architectural requirements, standard risk control, and
other novel security measures and policies from credible institutions, must be put in place [64].</p>
    </sec>
    <sec id="sec-9">
      <title>3.1.3. Threats to data privacy</title>
      <p>Data privacy in distributed DTs is one of the most alarming threats. The DT paradigm presents a
significant security concern regarding data privacy, primarily due to its emphasis on safeguarding the
proprietary information stored within its servers. Entities capable of infiltrating breached servers or
associated infrastructures may extricate sensitive data, including but not limited to login information,
setups, services, and other processes. They can harvest data for the purpose of industrial cyber espionage
or discover primary weaknesses in DTs to enhance data penetration or other components. Threats
related to software attacks and privacy leakage are some of the vulnerabilities surrounding data privacy.
In Figure 3, Layers 2 and 3 depict such threats within the DT platform.</p>
    </sec>
    <sec id="sec-10">
      <title>3.1.4. Threats via authentication and access control</title>
      <p>DTs generally handle sensitive and confidential data, which makes them a desirable target for
attackers with the intent of gaining illegal access to the systems. This action may also be undertaken
with the purpose of covertly acquiring data for other malicious intents, such as withholding critical data
for financial reasons and impersonation [65]. Consequently, the use of weak access restrictions or the
lack of authentication and access control mechanisms in distributed DT will lead to several other
vulnerabilities, including software attacks, privilege escalation, and different tampering scenarios.
These threats also cover a large attack surface in Figure 3, particularly from Layer 1, Layer 2
(computing infrastructure), and Layer 4 (HMIs). Figure 4 depicts the threat landscape in a distributed
DT paradigm.</p>
    </sec>
    <sec id="sec-11">
      <title>3.1.5. Threats to digital tools and services</title>
      <p>The IT and OT realms provide the backbone through which data exchange between different tools
and services is carried out within the DT paradigm. This brings yet another large attack surface to the
DT. Any form of threat that intercepts the edge connecting both realms will be devastating. Moreover,
ensuring the physical security of OT devices is crucial since they are susceptible to potential harm,
destruction, or theft by adversaries. The real physical replica of the DT also utilizes various
technologies, such as tamper-proof and tamper-resistant hardware, to safeguard their data against
unauthorized manipulation [66]. However, it still remains feasible for attackers to penetrate and modify
the data via rogue CPS/IIoT, digital threat tampering, man-in-the-middle and poisoning attacks, or by
leveraging other flaws within the infrastructure, applications, and communication channels. By
monitoring and assessing various physical traits, such as processing times, supply flow, and various
production channels, the attacker can also exploit confidential information through a physical device.
This threat also exhibits a large attack surface, especially in Layers 1, 2, and 4, indicated to Figure 3.</p>
    </sec>
    <sec id="sec-12">
      <title>3.1.6. Threats via network connectivity</title>
      <p>
        Given that the distributed DT logic is distributed across a whole network of computing ecosystems
through clouds, fog, and at the edge, rogue DT servers and devices can function as an attack surface
[67], allowing DT communication to flow through. Similarly, such servers whose nodes carry a portion
of the logic have the potential to introduce inaccuracies within the information under processing, modify
or breach the storage systems under monitoring, and alter the generated knowledge or representation
meant for the end user [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]. Common vulnerabilities to network infrastructures due to the high
interconnectivity include software attacks, man-in-the-middle attacks, and manipulation-based attacks
(DoS and DDoS). Figure 3 delineates the presence of these threats in all layers of the DT platform.
Additionally, Figure 5 depicts the information flow characterization between cyberspace, DT, and
cyberthreats.
      </p>
      <p>Cloud-fog-edge computing, Visualization</p>
      <p>Cyber physical systems, Protocols, Control</p>
      <p>systems, Communication technology</p>
    </sec>
    <sec id="sec-13">
      <title>Strategies for mitigating cyberthreats in distributed DT</title>
      <p>Similar to other new technologies and infrastructures, distributed DTs face the same
cybersecurityrelated risks. They need to effectively tackle the various types of cyberthreats mentioned earlier and
establish strong security regulations and frameworks. A complete cybersecurity strategy and plan
should include security measures in several facets, including secure network connectivity, software
security, hardware security, secure communication channels, etc. All with the aim of ensuring data
protection, network protection, access control, identity control and authentication, cloud protection,
databases and cloud infrastructure protection, mobile protection, endpoint protection, and training of
personnels/OT systems.</p>
      <p>In the following, we provide defense mechanisms against cyberthreats that DTs are susceptible to.
These mechanisms are based on recent novel techniques put forward by researchers and cybersecurity
experts to mitigate different cyberthreats in DT.</p>
      <sec id="sec-13-1">
        <title>Digital Twin</title>
        <p>Data</p>
        <p>Information</p>
      </sec>
      <sec id="sec-13-2">
        <title>Cyberthreats</title>
        <p>OT</p>
      </sec>
    </sec>
    <sec id="sec-14">
      <title>3.2.1. Recent defense mechanisms</title>
      <p>
        Authentication and access control: Strong authentication and access control mechanisms are
essential in the DT to ensure that access is only granted to authorized entities. Through the
implementation of authentication, users will have the ability to safely access various data processing
services within the DT paradigm through secure login credentials. Also, considering the complex
nature of distributed DTs in terms of their heterogeneous interconnectivity with other technologies,
authentication and access control will provide a secure and seamless DT interaction with other
interfaces and processes. Some novel state-of-the-art data authentication mechanisms in the
communication space of DT in different manufacturing sectors include the works in [68-72]. The
authors mainly emphasize privacy-preserving authentication between different communication
scenarios. Pervez et al. [69] specifically adopt the concept of ‘Verifiable Credentials’ in their
proposed framework to ensure the verifiable authenticity of Digital Twin data by encompassing
provenance, transparency, and dependability via the use of verifiable representation. Likewise,
Chen et al. [68] and Jiang et al. [73] utilize framework models at different layers of their
architecture, which provide flexible support for internal expansion of the DT and enable multi-DT
connectivity to more accurately represent intricate systems within the digital realm. This also
ensures the proper management of many internal and external interfaces within the ecosystem. In
terms of access control, techniques such as [74] and [75] provide frameworks for preventing
unauthorized access to multiple systems that exist together within the DT paradigm. Other access
control-based blockchain schemes include [76, 77]. Both techniques ensure the secure exchange of
DT data while preserving privacy. This gives adequate protection and access to resources by
granting the right privileges to the right parties involved.
2. Real-time monitoring and incidence response: The real-time virtualization and monitoring of
security-related events generated by DTs and the prompt response to incidents are essential steps
in ensuring the optimal functionality and security of the DT platform. This is to guarantee early
detection of potential threats, exploits, and vulnerabilities within the entire ecosystem. The security
operation center (SOC) [78] is the centralized entity responsible for the real-time monitoring and
detection of cyberthreats on IIoT platforms. Security Information and Event Management systems
(SIEM) are crucial tools utilized in SOCs. They gather critical events from various sources within
the entire DT network, standardize the events to a uniform format, and store the standardized events
to promptly detect illicit activity [78]. To enhance proactive security measures and improve
situational awareness in DTs, SIEMs and SOCs can additionally include Cyber Threat Intelligence
(CTI) protocols [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ] to monitor and control the exchange of critical security information. Moreover,
the efficacy of these monitoring methods also relies on the ability of their data analytics approach
in terms of collection, analysis, and providing prompt and reliable feedback to reactive systems
[78].
3. Network security: The network interconnectivity between distributed DT layers and other IIoT
devices within the DT ecosystem is considered crucial as it comprises a large attack surface and
vulnerabilities. Therefore, it is imperative to adopt efficient network defense mechanisms and
incorporate them into all associated technologies within the ecosystem. Some of these technologies
are covered in [79, 80], where priority is given to architectural design that supports network security
by design. In addition, as good practices and preventive measures, techniques such as network
segmentation [73, 81] and network configuration [82, 83] should be adopted and implemented
within the ecosystem. All of these will serve as the first line of defense in the event of cyberthreats
within the network infrastructure.
4. Data security: Data serves as the core basis on which other platforms, services, and technologies
operate in the digital domain, let alone in distributed DT. As such, data security should be an
integral part of every DT platform, given that DT’s entire functionality revolves around data use.
In fact, without sufficient data availability, the concept of DT would not have existed. With this in
mind, researchers and cybersecurity experts have proposed different frameworks [69, 84, 85] with
the aim of securing data in various DT scenarios. Consequently, encryption techniques [86] also
provide efficient protection to data against malicious manipulations, which should be adopted in all
DT platforms. Moreover, all the other security measures and defense mechanisms, such as
authentication, access controls on specific data, periodic real-time monitoring audits and incidence
response, as well as network security, also provide protection for the DT data.
5. Privacy-preservation: Data privacy is another important requirement of DT data protection. It
primarily concerns the rights and privileges of users in relation to their critical data and personal
information. Therefore, considering the massive processing and analysis of data performed in DT,
it is paramount to ensure all crucial data within the DT ecosystem is given the utmost
privacypreserving protection. Recently, technologies such as federated learning [87, 88], blockchain [76],
secure multi-party computation [89], and pseudo anonymization and differential privacy [90] have
been proposed to mitigate privacy risks in distributed DT. The combination of federated learning
and blockchain empowered technologies [91, 92] has also seen a progressive surge. Li et al. [92]
leverage access point model learning and directed acyclic graph (DAG) to exploit blockchain for
secure model updates. This ensures efficient DT architectural design while preserving data privacy.
6. Training of personnels: Personnel training on how and what to do in case of cyber-attack is a
crucial aspect to protecting IT and OT systems against cyber threats. Extensive cybersecurity
training should be given to personnel who are responsible for the development, operation, and
maintenance of IT and OT tools and systems. This guarantees a workforce that is both vigilant and
well-informed. Moreover, personnel in the IT realm should be familiar with all tools and
technologies used by their OT counterparts and vice versa. Also, when creating the training
programs, it is advisable to employ activity monitoring rules to assess the level of expertise,
competency, and abilities in the proper use of the IT and OT tools and technologies, as well as the
corresponding cyber threats. This ensures synergy in both their activities and knowledge of cyber
threats.
      </p>
    </sec>
    <sec id="sec-15">
      <title>4. Key takeaways</title>
      <p>Generally, our review yields the following findings that are worth further research: (a) The security
of OT systems is not sufficiently investigated in both academia and industry. (b) Security guidelines,
policies, and preventive methods are often ignored. (c) Inadequately adaptive security measures lose
their efficacy over time. (d) DTs for cybersecurity and (e) Cyber-immune systems for DTs are worth
exploring.
a) OT systems security needs more research: Considering the security of OT is not as mature as
that of IT [30], present security measures [34] are insufficient in providing the required protection
due to the integration of OT systems with diverse characteristics and interconnectivity with the IT
industrial systems. Therefore, OT security solutions are worth investigating in both academia and
industry.
b) Security policies and preventive measures are often ignored: Implementing security policies is
the first and most viable approach to mitigating attacks and human errors. Recent research,
however, indicates that academia places a strong emphasis on intrusion detection systems after an
attack occurs, but lacks focus on studies based on security policies before an attack occurs [30].
Therefore, it is important that the research community also focus on preventive measures through
security policies before an attack happens.
c)</p>
      <p>
        Cybersecurity solutions that are not adaptable become invalid: Traditional industrial systems
were constructed with a fixed architecture that consisted of modules designed to operate reliably
for a specific period of time without substantial modifications. Nevertheless, the IIoT architectures
exhibit dynamism since they include different technologies, and such dynamism comes with a
plethora of solutions, some of which are novel and efficient [
        <xref ref-type="bibr" rid="ref5">5, 76, 88, 93</xref>
        ]. However, as the race
between adversaries and cybersecurity experts continues, all solutions that are not adaptable
become invalid over time. Therefore, it is important to incorporate recent solutions to emerging
technologies such as DT.
d) DTs for cybersecurity: Recently, researchers have started exploring the notion of using DT for
cybersecurity, coined the Cyber Digital Twin [93-96], which is gaining momentum. This concept
highlights that industries can use DTs to identify and eliminate vulnerabilities in their systems by
generating virtual replicas for the purpose of conducting security assessments, thereby enhancing
cybersecurity by responding to cyberthreats in a manner that closely resembles a real secure system.
To this end, we believe that establishing a novel research direction focused on investigating the
potential of DTs to serve as mechanisms for bolstering the security of different critical
infrastructures is imperative.
e)
      </p>
      <p>
        Cyber-immune system for DT: As the integration of OT and IT exposes industrial technologies
to more vulnerability and increases the attack surface, the implementation of a robust cybersecurity
system that aims to replicate the adaptive immune system of humans is an effective approach to
safeguarding against existing and emerging cyberthreats [97]. Cyber-immune systems can survive
various types of attacks, particularly those caused by novel malware and viruses that cannot be
detected by traditional cybersecurity solutions [98, 99]. As a result, using cyber-immune systems
could be the key to making sure that data processing on the DT platforms is more secure than ever,
considering the immense vulnerability and large attack surface of DTs.
5. Challenges
a) Expansive attack surface: Due to the expansive interconnectivity landscape covered by
distributed DT, its attack surface happens to be one of the largest (if not the largest) among the
recent novel technologies. Also, the creation of a DT visual replica essentially multiplies the
potential attack surface, as adversaries can target either the physical system or its digital
counterpart. This poses a huge challenge to researchers and cybersecurity experts in reaching the
required protection, considering the immense performance-security tradeoff this expansive attack
surface could lead to.
b) Lack of standardized assessment criteria: Cybersecurity research for distributed DT and other
novel technologies lacks a standard framework for assessing vulnerabilities. Most proposals and
security measures focus on adopting conventional security solutions. However, evaluating the
framework implementation of DT through the modeling of attacks and defenses while studying the
system for a certain duration may provide the most accurate assessment. The major drawback,
however, is that it is not permissible to interfere with the real system owing to the crucial nature of
its activities and the need for constant supervision. Even though some solutions [
        <xref ref-type="bibr" rid="ref8">8, 70, 86</xref>
        ] are
suggested to overcome this limitation, their effectiveness is uncertain owing to the absence of an
assessment framework [86].
c) Lack of trained personnels: Considering the expansive nature of cybersecurity due to its
integration with other security domains, it is difficult for a single expert to tackle the plethora of
security challenges. Therefore, a unanimous collaboration of experts in various security domains is
imperative. In addition, the OT stakeholders also lack the relevant training and security awareness
for managing the new IT technologies integrated within the OT systems due to their acquaintance
with the traditional OT tools and systems [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ]. Therefore, it is important to establish training
programs that integrate both domains. An effective approach to promoting learning in both IT and
OT might include implementing frequent training programs that use tailored and integrated
instructional methods, specifically using cyber-range concepts [100-102].
d) Interoperability: The interoperability of distributed DTs is the ability of technologies to efficiently
communicate information without restrictions across different DTs and their integrated systems,
both in physical and virtual spaces [103]. Such interoperability encompasses several elements,
including hardware, software, algorithms, interfaces, operating systems, etc., and one major
research obstacle towards interoperable DTs is achieving comprehensive cross-chain interoperable
processes [86]. Therefore, a collaborative effort from both industries and academics is required to
establish regulations and standards toward streamlined distributed DT operability.
      </p>
    </sec>
    <sec id="sec-16">
      <title>6. Conclusion</title>
      <p>A distributed DT is built upon the integration of several technologies, including CPS, IIoT, AI,
edge computing, big data, etc. The convergence of these novel technologies, in tandem with the inherent
DT interconnections and data sharing with its corresponding physical counterpart, gives rise to several
security concerns that have not been adequately investigated. To propose efficient solutions, this paper
provides an in-depth discussion and analysis of the impact and significance of cybersecurity in
distributed industrial DTs. The work first examined the importance of DT and cybersecurity in industry,
followed by potential cyberthreats in industrial DTs. The analysis of threats within the large attack
surface of the DT platform is covered in terms of the four functional layers of the DT. Additionally, the
defense mechanisms for mitigating such threats are discussed in terms of recent technical approaches.
Finally, we provide important key takeaways toward the realization of a secure and privacy-preserving
industrial DT as well as challenges to existing DT security. Future work will focus on exploring the key
takeaways of our findings to uncover efficient methods of securing key architectures for distributed
DTs in the industrial domain.</p>
    </sec>
    <sec id="sec-17">
      <title>7. Acknowledgements</title>
      <p>The authors extend their thanks for the funding received from the ONE4ALL project funded by
the European Commission, Horizon Europe Programme under the Grant Agreement No. 101091877.</p>
    </sec>
    <sec id="sec-18">
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