Air-gapped OT security. How to protect an isolated network.
Air gap misconceptions, layered security and Zero Trust inside an isolated network
An air gap keeps remote attackers off an OT network. People, laptops and USB drives still cross it during routine maintenance. This guide shows where isolated networks get bridged, how an air gap compares with layered security, and which Zero Trust controls work with no internet at all.
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Is an air gap enough to secure an OT network?
No. An air gap is a strong control against remote attacks, and it is only one control. It removes the network path from the internet to your machines. It does nothing about the paths people create during routine work: a USB drive with a firmware update, a vendor laptop plugged into a switch, a maintenance modem, an engineering workstation with two network cards.
Inside the gap, many OT networks are flat. Anyone who reaches one machine can reach the rest, often with shared accounts and no log of who did what.
So keep the air gap, and add three things inside it:
- Control every path that crosses the gap
- Tie every session to a named person
- Keep a local record of what each person did
What is air gapping?
Air gapping means running a network with no physical or logical connection to the internet or to other untrusted networks. Data enters and leaves only by hand: removable media, a transfer station, or a person carrying a laptop across. Defense sites, classified facilities, substations and some water and manufacturing plants use it for their control systems.
Air gapping is a design choice about connectivity. It is not, on its own, an access control, a logging system or a segmentation scheme. Those still have to exist inside the gap.
Four air gap misconceptions in OT security
Each of these comes up in plant reviews and audits. Each one is partly true, which is why it lasts.
Physical isolation equals security.
Isolation removes one class of attack: a remote connection from the internet. Stuxnet showed in 2010 that malware can cross an air gap on a USB drive and reach controllers that were never online. Isolation lowers exposure. It does not decide who touches a machine once they are inside.
A disconnected network has no cyber threats.
Insiders, infected removable media and compromised supplies all reach isolated networks. A technician who copies a project file from an office PC brings whatever that PC carried. A disgruntled employee needs no internet connection at all.
The gap is still there because we built it.
Air gaps erode. A support modem, a cellular router added for a vendor, or a historian with a second network card can quietly connect the isolated side to the business network. Many sites have not checked in years.
An air gap fits every system.
Some systems need data out in near real time: production reporting, energy management, remote diagnostics. Forcing those flows onto USB drives adds manual work and creates more removable media traffic, which is the main way isolated networks get infected.
The agencies say the same. In Adapting Zero Trust Principles to Operational Technology (April 2026), CISA and its partner agencies advise against buying components or systems that rely on air-gapping or segmentation alone for their security, because attackers exploit the false sense of isolation. Our explainer on the CISA Zero Trust OT guide covers what that means for on-premise teams.
How do attackers cross an air gap in practice?
They use the paths the plant already uses to get work done. Each one below exists for a good operational reason, so the goal is to control it.
Removable media
USB drives carry PLC programs, firmware, recipes and log exports. This is the path Stuxnet used. Scan media on a dedicated station before it enters, and limit which machines accept USB at all. For routine file transfer, a fixed transfer station or a one-way data diode is easier to control than loose drives.
Vendor and maintenance laptops
A vendor technician plugs a laptop into the control network to service a machine. The laptop has been on other customers' networks and on the internet. Give vendors a named, time-limited session to one machine instead of a free port on the switch.
Maintenance and remote support links
Dial-up modems, cellular routers and remote support tools are often added for one vendor and never removed. Each is a live path through the air gap. List them, close the ones nobody uses, and route the rest through a point where access is authenticated and logged.
Dual-homed hosts
Engineering workstations, historians and HMI servers sometimes have one network card on the control side and one on the business side. That host is the air gap's real boundary, and it is usually a general-purpose Windows machine.
Supply chain
Controllers, drives and software arrive with firmware already loaded. Check firmware integrity and vet suppliers, as NIST SP 800-161 recommends for supply chain risk management, before new equipment joins the isolated network.
Malware detection updates
Antivirus and IDS signatures go stale without the internet. Bring signed signature updates in on scanned media on a fixed schedule, and add behavior-based monitoring, which does not depend on signatures to flag unusual traffic.
Air-gapped vs layered security architectures
A layered architecture, also called defense in depth, places several independent controls between an attacker and a machine. An air gap can be one of those layers. The table shows what each approach covers when it is used alone.
| Question | Air gap alone | Layered security |
|---|---|---|
| Remote attack from the internet | Blocked while the gap holds. | Blocked or filtered at several boundaries: firewall, DMZ, access gateway. |
| Infected USB drive or laptop | Not covered. The gap is already crossed. | Media scanning, port control and access control limit what the device reaches. |
| Movement between machines inside | Not covered. Most isolated networks are flat. | Segments and conduits limit each machine to the peers it needs. |
| Who did what, and when | Usually unknown. Shared accounts and no central log. | Named sessions and local audit logs answer it. |
| Patching and signature updates | Slow and manual. | Still manual for isolated zones, planned through a controlled transfer path. |
| Data to IT and reporting | By hand, on removable media. | Through a DMZ or a one-way link with defined flows. |
| Operating effort | Low tooling, high manual work. | More components to run, less manual handling. |
The two approaches combine well. Keep the most critical zone isolated, and build the layers inside and around it: a DMZ for the data that must leave, zones and conduits as described in IEC 62443, and access control in front of each machine. The Purdue model page covers the zone layout that most layered OT designs start from.
Alternatives to a full air gap for systems that need connectivity: a one-way data diode for outbound data, an industrial DMZ with no direct path from IT to OT, and an access gateway that authenticates each person before they reach a machine. Each keeps most of the isolation benefit and gives the plant a controlled path in place of an improvised one.
How does Zero Trust work in an air-gapped OT network?
Zero Trust is a set of rules, so it works offline: verify each person, grant the least access needed, and log every session. What breaks is the tooling. Most Zero Trust products assume a cloud control plane, and an isolated network cannot reach one.
What does not work without the internet
Cloud identity providers
Azure AD (Entra ID), Okta and Google Workspace authenticate users over the internet. Behind an air gap there is nothing to call.
Cloud-managed firewalls and SASE
Products that route traffic through cloud inspection points, or pull policy from a SaaS portal, have no route out.
SaaS SIEM
A cloud SIEM cannot receive logs from a disconnected network. The audit trail stays local or does not exist.
Endpoint agents
Most PLCs and HMIs cannot run an agent. The ones that can usually need a cloud service for updates and policy.
What works inside the gap
Local identity and offline MFA
A local directory and an identity gateway on site. Time-based one-time passwords (TOTP) work offline: the token and the gateway share a secret and a clock, with no external call.
Access control at the network layer
A gateway in front of the machines checks each session before it reaches a PLC or HMI. Nothing is installed on the machine.
Local session logs
Each connection is recorded on site with the user, time, source and target, and exported to a local SIEM by syslog.
Segments inside the flat network
Group machines and allow only the paths each group needs, so one infected laptop does not reach the whole plant.
Can secure access operate in a segmented or air-gapped OT network? Yes, when every component runs on site. Identity is the part teams most often get stuck on; user identity and access in air-gapped environments covers on-premise identity management, offline authentication methods and the NIST SP 800-171 and CMMC angle in detail. The MFA for OT guide shows where the second factor belongs, and the Zero Trust for OT hub covers the wider architecture.
How to secure an air-gapped OT network, step by step
Inventory every crossing path
List every USB-capable machine, vendor connection, modem, cellular router and dual-homed host. This list is the real boundary of your air gap.
Outcome
A written map of how data and people enter the isolated network.
Control each crossing
Scan media at a fixed station. Replace open switch ports for vendors with named, time-limited sessions. Remove links nobody uses.
Outcome
Every remaining path is known, approved and owned by someone.
Add identity and access inside
Put an on-site gateway in front of the machines. Each person signs in with MFA and reaches only the machines their task needs.
Outcome
No shared accounts on the path to a PLC or HMI.
Log and review locally
Keep session logs on site, export them to a local SIEM, and review them on a schedule. Plan signature and software updates on the same transfer path.
Outcome
You can answer who did what, on which machine, and when.
Access Gate runs entirely inside the air gap
Trout Access Gate is an on-premise appliance with no cloud and no call-home. It is managed from a local web console, with no activation server and no online license check. It authenticates each person, applies deny-by-default access in front of OT machines without installing anything on them, and keeps audit logs on the appliance for export to your own SIEM by syslog or file. Signed updates arrive by USB or an internal share and are verified locally before they are applied.
Questions about air-gapped OT security.
No. An air gap blocks remote attacks from the internet, but removable media, vendor laptops, maintenance links and dual-homed hosts still cross it. Inside the gap, many OT networks are flat and use shared accounts. Keep the gap and add access control, named sessions and local logging inside it.
Air gapping means running a network with no connection to the internet or other untrusted networks, so data moves in and out only by hand, on removable media or through a transfer station. It reduces remote exposure. It is not an access control or a logging system on its own.
Stuxnet reached isolated systems on infected USB drives, then spread to the engineering software and controllers inside the facility. It is the best-known example of removable media bridging an air gap.
An air gap is one control: isolation from outside networks. Layered security, or defense in depth, combines several independent controls such as a DMZ, segmentation, access control and monitoring, so one failure does not expose every machine. An air gap can be one layer of a layered design.
Yes. Zero Trust is a set of principles, so it works offline when every component runs on site: a local identity source, offline MFA such as TOTP, a gateway that checks each session in front of the machines, and local logs. Cloud identity providers, cloud-managed firewalls and SaaS SIEMs do not work behind an air gap.
Bring signed signature updates in on scanned media through a fixed transfer path on a regular schedule. Add behavior-based or anomaly-based monitoring, which flags unusual traffic without needing fresh signatures.
Only in part. Isolation helps with boundary protection, but NIST SP 800-171 still requires access control, multi-factor authentication, audit logging and encryption inside the system. An air-gapped CUI environment needs those controls running on site.