[Security Architecture] Operational technology often relies upon aging command, control, and telemetry subsystems that were created with the design assumption of:

Aging Operational Technology (OT) and Industrial Control Systems (ICS) were fundamentally designed with the assumption that they would operate in physically and logically isolated environments, often referred to as being "air-gapped." Security was predicated on this physical separation from other networks, such as corporate IT networks and the internet. Consequently, these legacy systems were not built with robust cybersecurity controls, as external threats from interconnected networks were not part of the original threat model. The convergence of IT and OT has since invalidated this core design assumption, exposing these systems to a wide range of modern cyber threats.

B. communicating over distributed environments: While some OT systems are geographically distributed (e.g., SCADA), their communication was assumed to be over private, dedicated links, not open, untrusted distributed environments like the internet.
C. untrustworthy users and systems being present: This reflects a modern "zero trust" approach. Legacy OT systems were designed with the opposite assumption: that any user or system with network access was inherently trusted.
D. an available Ethernet/IP network stack for flexibility: Many aging OT systems predate the widespread adoption of Ethernet/IP and rely on proprietary serial protocols (e.g., Modbus RTU over RS-485), making this a more modern feature, not a foundational assumption.
E. anticipated eavesdropping from malicious actors: Because the systems were assumed to be isolated and physically secure, eavesdropping was not a primary design concern, which is why many legacy OT protocols lack encryption. ---

1. National Institute of Standards and Technology (NIST). (2015). Guide to Industrial Control Systems (ICS) Security (NIST Special Publication 800-82

Revision 2).

Section 2.1

Page 2-1: "Traditionally

ICS were isolated systems running proprietary control protocols

using specialized hardware and software. Widespread connectivity to IT systems was not a design requirement..." This directly supports the assumption of isolation.

Section 2.3

Page 2-6: "ICS had been perceived as being isolated from other networks and the threats that were common in IT networks..." This reinforces that the design did not account for external threats.

2. Cybersecurity and Infrastructure Security Agency (CISA). (2020). Securing Industrial Control Systems: A Quick Start Guide.

Page 1

Introduction: "For many years

industrial control systems (ICS) were isolated

operating in separate networks using specialized hardware and software. Security was not a primary design consideration. Today

however

these systems are increasingly connected to other networks

including corporate networks and the Internet." This highlights the shift from the original isolated design assumption.

3. Knowles

W.

Prince

D.

Hutchison

D.

Disso

J. F. P.

& Jones

K. (2015). A survey of cyber security management in industrial control systems. International Journal of Critical Infrastructure Protection

9

52-80.

Section 2.1

Page 54: "Historically

ICS were isolated from other networks and considered to be secure by means of this ‘air-gap’... As a result

security was not a priority in the design of these systems

and they were developed with a focus on reliability and availability." This academic source confirms that isolation was the assumed security mechanism. (DOI: https://doi.org/10.1016/j.ijcip.2015.02.001)