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Replace broad remote network admission with access to specific OT systems, but do it in reviewed stages—not with a plant-wide cutover. First map users, assets, required communications and operational hazards. Then introduce a hardened jump host in an OT DMZ, enforce identity and session controls, and allow only approved paths to approved assets. Have operations and safety owners validate each change before expanding it. This reduces unnecessary reach; it cannot guarantee a migration with no interruption.
What changes when remote access becomes identity-first?
A VPN can authenticate a user and encrypt a connection without limiting that user’s reach once connected. The concern with a “flat VPN” is broad network admission, not that every VPN is inherently insecure: some VPN deployments already apply strong segmentation and access controls. The goal is to ensure that establishing a tunnel does not, by itself, grant broad access to the plant network.
NIST describes zero trust as removing implicit trust based solely on network location and protecting individual resources rather than treating network segments as trusted by default. In OT, that principle must work alongside network boundaries and the requirements of physical processes. See NIST SP 800-207, Zero Trust Architecture, and the NIST Guide to Operational Technology (OT) Security, SP 800-82 Rev. 3.
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|---|---|---|
| Broad VPN access | A connection to a network or network segment; reach depends on the VPN and surrounding controls. | May be enforced at the VPN, firewall, or other boundaries. A VPN does not necessarily provide per-resource restrictions. |
| Identity-first remote access | A named user’s request to reach a defined resource for an approved purpose and period. | Identity and session controls govern the request; firewalls and segmentation constrain the network paths that can carry it. |
This is an architectural comparison, not a claim that every VPN has the same capabilities. Identity controls and network controls solve related but different problems: one governs who may request access, while the other limits where traffic can flow.
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Start with the plant, not the access product
Before changing remote access, establish what must remain reachable, by whom, and under what operating conditions. OT equipment may be legacy, difficult to patch, or sensitive to changes; cyber actions can affect physical processes, and availability requirements can constrain testing and maintenance. CISA and its partners emphasize adapting security to OT rather than disrupting essential operations in their April 29, 2026 OT zero-trust guide.
- Inventory assets and owners. Record the systems that may be accessed remotely, their management authorities, criticality, location, support arrangements, and known constraints.
- Identify people and purposes. List employees, integrators, vendors, and other remote users; note the business purpose, required destination, and expected maintenance or support window for each.
- Map necessary communications. Document the destinations, protocols, and data flows needed for legitimate work. Identify which communications cross levels, zones, or site boundaries.
- Assess operational consequences. Ask operations and safety owners what could happen if a connection is blocked, delayed, altered, or left open. Identify critical functions and the conditions under which remote work is safe.
- Include the people who operate and support the system. OT, IT, cybersecurity, engineering, operations, procurement, safety, vendors, and integrators may each hold information needed to set a workable policy.
NIST recommends grouping OT components using factors such as management authority, trust, criticality, data flow, and location before applying isolation devices. Purdue and ISA-95 are among possible organizing models, not mandatory designs. The CISA-led guidance likewise emphasizes visibility, identity and access management, supply-chain considerations, and collaboration.
Use an OT DMZ jump host as a controlled entry point
A practical target pattern is for the remote user to authenticate to a hardened jump host in the OT demilitarized zone (DMZ), then reach only the authorized system through a narrowly permitted path. CISA strongly recommends a hardened jump host in the OT DMZ as the sole remote entry point for legacy networks, to add authentication and enforce segmentation. NIST describes a DMZ as a possible enforcement boundary and recommends limiting allowed communications between adjacent levels, tiers, or zones.
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- Authenticate the remote user. Require multifactor authentication for remote privileged access. Use named accounts where the equipment and support model allow them.
- Approve a defined request. Specify the user, purpose, destination, permitted work, and time window. Use just-in-time access when limiting duration and lateral movement does not compromise safety or system integrity.
- Connect to the hardened jump host. Place it in the OT DMZ, patch it regularly, apply approved hardening, and monitor it continuously, as CISA recommends.
- Permit only the required next hop. Configure boundary controls so the session can reach the approved asset and service, rather than treating the jump host as a route to the wider OT network.
- Capture and review session evidence. Consider session recording, enhanced auditing, and anomaly detection; export logs outside the OT network without creating a bidirectional control path back into it.
Where legacy equipment still relies on shared credentials, protect those credentials in a vault where practical, rotate them when feasible, monitor their use, and maintain a controlled break-glass process. Emergency access should be monitored and governed, not left as an untracked parallel route.
NIST notes that remote OT access should be justified, limited to business need, and must not circumvent safety or security controls. It lists several possible mechanisms—including RDP or SSH through firewall rules, screen sharing, modems, and VPNs—while emphasizing that secure connection procedures remain necessary whatever technology is used.
Migrate in reviewed stages
No general guide can prescribe a sequence that is safe for every plant. The steps below are a risk-informed implementation approach based on the cited OT security and testing principles; they are not a certified no-stop procedure. Use the site’s management-of-change process and have operations and safety owners approve the plan.
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- Establish the current-state baseline. Verify legitimate users, vendors, destinations, protocols, work windows, existing routes, and emergency access. Resolve unknown or unowned connections before treating them as unnecessary.
- Design the replacement boundary. Define DMZ placement, jump-host administration, identity and approval rules, allowed network flows, logging destinations, and the path for emergency use. Document hazards and required operating procedures.
- Test outside live operations. Check compatibility with legacy operating systems, engineering tools, vendor workflows, and relevant protocols in a representative non-production environment. Coordinate modifications with vendors or integrators when needed. NIST cautions against using live operational systems to test modifications.
- Introduce one well-understood use case. Start with a limited group of users and a defined set of destinations. Observe whether approved work succeeds, whether prohibited paths are blocked, and whether the controls affect system performance or operator work.
- Review, adjust, and expand deliberately. Have OT operations and safety owners assess the results. Update policies and procedures before adding users, vendors, systems, or access types.
- Prepare recovery before removing the old route. Keep an approved rollback and emergency-access method, verify backups and configuration records around changes, and agree on a change window. Retire the broad route only after the replacement, operator procedures, monitoring, and emergency controls have been validated and accepted.
Remote access may be necessary for distributed infrastructure, but it also creates risk. CISA’s April 29, 2026 announcement of the joint OT zero-trust guide urges resilience without jeopardizing mission-critical operations. That is a design priority—not a guarantee that every migration can avoid interruption. A site may need to defer a change until it can be tested and scheduled safely.
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Monitoring method is a site-specific compatibility and visibility decision. CISA distinguishes endpoint-agent monitoring from agentless passive monitoring; neither is a universal answer.
- Endpoint agents may provide useful visibility, but require compatibility testing and can affect equipment warranties. Confirm vendor support and test on representative systems before deployment.
- Passive monitoring avoids installing an agent on the endpoint, but may not reveal abuse of a remote session until malicious commands begin. Do not treat passive visibility as proof that a session is safe.
- Session controls and operational response should include a tested way for authorized operators to disconnect or disable remote access without impairing OT operations. Define who can act, how the decision is recorded, and how service is restored.
Evaluate the whole access path, not just the login screen
When assessing a remote-access platform or architecture, examine whether it fits the plant’s equipment and operating model. A product category alone does not establish that a particular offering meets these needs.
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- Compatibility: Can it work with required legacy operating systems, vendor engineering tools, and OT protocols? What testing, maintenance, or warranty implications do agents create?
- Identity and privilege: Can it integrate with organizational identity controls, enforce MFA for privileged access, separate roles, and support defined approvals and time limits?
- Scope of access: Can policy restrict users to specific resources and services, while network controls independently enforce allowed flows between zones?
- Session evidence: What can be recorded or audited? Does it provide useful command visibility or alerts, and can logs be exported without a return control path into OT?
- Availability and recovery: What happens if the access service or its management plane is unavailable? Is there an operator-approved disconnect, emergency process, and tested rollback?
- Deployment and operations: Where does the system sit relative to the DMZ and firewalls? What management interfaces are exposed, and what support, patching, and change-management work will the site own?
Industrial firewall appliances can help enforce segmentation and isolation at network boundaries; NIST identifies firewalls as a commonly used boundary-protection measure. They do not, by themselves, provide identity-first remote access. The access design still needs identity, privilege, approval, and session controls.
What the guidance does—and does not—establish
NIST SP 1800-35, published in 2025, documents 19 example zero-trust architecture implementations developed with 24 technology collaborators. Those examples can inform zero-trust implementation, but they are not OT validation or measured evidence that a plant migration will preserve uptime. Neither the cited guidance nor those example counts establish a universal migration duration, downtime avoided, or incident reduction for OT.
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