Ensuring OT Cybersecurity During Maintenance Windows of Plants and Sites

Across the many interactions that I have had with OT operators, one topic that has consistently popped up is ensuring security during maintenance windows.

During such periods, critical systems may be taken offline for patching, updates, physical inspections, or simply recalibration. While essential from a reliability and compliance standpoint, maintenance windows present a unique convergence of increased human intervention, relaxed operational controls, and (usually) temporary bypassing of standard defenses. Adversaries know this, and they often time their intrusions, persistence activities, or lateral movement for these windows of opportunity.

In today’s blog we do a deep dive and learn why OT maintenance windows are a cybersecurity blind spot, the tactics adversaries use, and learn actionable measures that security and engineering leaders can take to mitigate risks.

Before we move forward, did you get a chance to peep into our blog on identifying OT asset visibility gaps in the beverage industry? In case you didn’t, you can read the post here.

Let’s dive in.

Why are maintenance windows turning into OT cybersecurity blind spots

As any manufacturer would tell you, maintenance activities are unavoidable in any industrial environment. But they bring forth cyber risks that differ significantly from day-to-day operations:

Temporary bypass or disabling of protection systems

• NDRs, firewalls, intrusion prevention systems, and even safety interlocks may sometimes be disabled temporarily for system testing.

• External contractors may be granted privileged access to PLCs or HMIs.

• Legacy devices may be patched directly from laptops that are not subject to the plant’s normal security hardening.

• Plant staff may use personal mobile hotspots to access the internet and download patches

Increased human presence

• Vendors, contractors, and OEM engineers may enter with their own equipment.

• Boundaries of credibilities blur, often these devices bypass strict allow-lists and carry unverified firmware or tools

• Chances of planet personnel being distracted are also high

Compressed timelines

• Maintenance windows are typically short, increasing the tendency to override security processes “just to get things done.”

• Pressure to minimize downtime creates opportunities for corners to be cut.

• Systems may be shutdown and rebooted without adhering to a security sequence

Visibility gaps

• OT monitoring tools may be powered down, tuned into maintenance mode, or blinded by large volumes of legitimate reconfiguration traffic. This creates a fog under which malicious activities can be initiated without detection.

Attractive for adversaries

Cybercriminals, APT groups, and even insiders know these are prime opportunities to:

• Deploy persistence mechanisms such as unauthorized loiterware.

• Install unauthorized remote access tools.

• Move laterally between IT and OT environments.

• Exfiltrate sensitive engineering data without triggering alarms.

• High risk commands can be fired on unprotected systems

Threat actor tactics during maintenance windows

To understand the threat, we must map adversary behavior to real-world OT operations. Using frameworks like MITRE ATT&CK for ICS, here are key tactics adversaries leverage during downtime:

• Initial access via contractor devices Plugging in laptops with outdated patches or malware-laden USB drives by an insider or a threat actor agent.

• Persistence: Installing malicious ladder logic in PLCs, often disguised as calibration scripts.

• Privilege escalation: Exploiting temporarily relaxed account provisioning to gain higher-level rights.

• Lateral movement: Pivoting between engineering workstations and OT assets when defenses are in maintenance mode.

• Impact activities: Tampering with safety instrumented systems (SIS) logic or modifying historian data to cause misconfiguration after restart.

Adversaries don’t need the maintenance window to achieve full disruption immediately, they often just need to deploy vectors that can open backdoors later.

Treat maintenance windows as high-risk operations

A robust IEC 62443-based OT cybersecurity program must recognize that maintenance equals high risk. This means designing governance, processes, and controls specifically tailored to these periods. Think of it as the “surgical theater” mindset: just as medical procedures involve heightened hygiene and strict protocols, so should OT maintenance involve heightened cybersecurity controls.

Deploy multi-level measures to secure maintenance windows

1. Pre-Maintenance Planning

Cybersecurity playbooks: Develop a maintenance Security Playbook aligned with IEC 62443 and NIST CSF.

· Define who approves temporary control bypasses, how risks are logged, and what compensating controls must be enabled.

Asset and patch validation

· Maintain an up-to-date OT asset inventory with firmware versions, patch levels, and known vulnerabilities.

· Validate patches in a lab or digital twin environment before plant-wide deployment.

Contractor and vendor vetting

· Require vendors to sign off on cybersecurity requirements before arrival.

· Mandate use of vendor access portals or secure remote access solutions with session recording.

· Validate contractor laptops against corporate baselines (AV, patching, encryption) before granting access.

Incident response, backup and recovery preparation

· Take golden image backups of PLCs, SCADA servers, and HMIs before maintenance starts.

· Store them in a secure, immutable repository in case rollback is needed.

· Keep incident response teams on standby

2. During Maintenance

Network segmentation and controlled access

· Isolate maintenance traffic in dedicated VLANs or zones.

· Apply strict firewall rules limiting contractor devices to only the systems they need.

Portable media control

· Enforce policies where USB devices are scanned in secure kiosks before use.

· Where possible, prohibit direct use of portable media, use secure transfer gateways instead.

Active monitoring

· Increase Network Detection and Response (NDR) sensitivity for anomalies in engineering protocols (Modbus, DNP3, PROFINET, etc.).

· Deploy a “red team mindset”: assume adversaries may attempt persistence, watch for unusual configuration changes.

Privilege management

· Provision temporary accounts for contractors, never allow sharing of permanent plant accounts.

· Implement time-bound, just-in-time access with automatic expiration after the window.

Strict change management

· Every configuration change should be logged with who, what, when, and why.

· Video or screen recording of maintenance sessions can act as an audit trail.

3. Post-Maintenance Hardening

Validation Testing

· Conduct integrity checks of PLC logic against baselines.

· Verify safety interlocks and alarms are functioning correctly after reconfiguration.

Threat Hunting

· Perform post-maintenance compromise assessments:

• Look for anomalous services on engineering workstations.

• Hunt for indicators of persistence in PLC memory.

• Review all logs for unapproved lateral movement attempts.

Forensics Readiness

· Archive logs, access records, and session data in tamper-proof storage.

· Prepare them for regulatory compliance or incident investigations.

Lessons Learned

· Conduct a post-mortem security review involving operations, engineering, and cybersecurity teams.

· Feed findings into playbook updates.

Building organizational resilience

Beyond technical controls, the ability to safeguard OT during maintenance windows depends on people and governance.

Cybersecurity Training for Maintenance Staff

• Train operators, engineers, and contractors on cyber hygiene, phishing, USB risks, remote access pitfalls.

• Use scenario-based tabletop exercises specifically simulating maintenance-related attacks.

Cross-Team Collaboration

• Encourage collaboration between OT engineers, IT security, and plant managers.

• OT staff understand process safety; IT staff understand adversary tactics, the synergy is critical.

Culture of Security Accountability

• Make it clear that cybersecurity is not an “IT add-on” but part of plant safety.

• Establish accountability: if a firewall is disabled, who signs off? Who ensures it is re-enabled?

Compliance Integration

• Align maintenance security practices with NIS2, IEC 62443-3-3, and local regulations (e.g., NIS2 in EU, OTCC in Saudi Arabia, India’s CEA guidelines or NERC CIP for North America).

• Demonstrate that secure maintenance is both a compliance and resilience requirement.

Advanced Measures for Mature Organizations

For organizations already implementing the above, here are advanced steps:

Run integrated cyberphysical measures to manage cyber and physical security. Ensure plants falling under maintenance windows are kept at a state of heightened alert.  

Adopt digital twins for patch testing

• Simulate maintenance scenarios in a digital twin to validate security before touching production assets.

Zero Trust for OT maintenance

• Extend Zero Trust Network Access (ZTNA) to OT environments, contractors authenticate continuously, with behavior-based trust scoring.

Behavioral analytics on engineering Changes

• Use machine learning to detect unusual programming patterns in PLC logic.

Immutable logging with blockchain or WORM

• Ensure maintenance logs cannot be altered by insiders or adversaries.

Supply chain security

• Validate firmware and patches from OEMs through cryptographic signatures.

• Monitor for compromised vendor updates (a tactic seen in SolarWinds-style attacks).

Case Studies

Triton/Trisis Malware (2017)

• Attackers compromised safety instrumented systems (SIS) during routine maintenance work, nearly causing catastrophic failure.

• Lesson: Even safety systems are not immune during maintenance.

Ransomware in European Manufacturing (2020)

• Attackers exploited open remote desktop sessions left active for vendor patching.

• Lesson: Temporary access must never be left unmonitored or persistent.

Asian refinery attack (2022, unattributed APT)

• Adversaries inserted persistence in PLC firmware during a planned shutdown, only detected months later.

• Lesson: Post-maintenance threat hunting is critical, not optional.

To sum it all, maintenance windows should not be viewed merely as downtime for equipment, they must be seen as high-risk operations for cybersecurity. Just as plants treat shutdowns as critical safety events, organizations must apply the same rigor to digital safety.

CISOs, plant managers, and OT security leaders must ensure that:

· Every maintenance window begins with a cybersecurity plan with incident response factored in.

· Every contractor or vendor enters with a clear security baseline.

· Every configuration change is audited, validated, and monitored.

· Every post-maintenance cycle includes threat hunting and lessons learned.

· A security expert is consulted for ensuring security during the window

By reframing maintenance as a pre-incident situation with heightened situational and risk awareness, organizations can deny adversaries one of their most exploitable opportunities. The result is stronger resilience, higher safety, and a plant that is truly secure, not just when running, but also when under repair.

Want to learn more about ensuring site security during maintenance windows? Talk to our IEC 62443 and NIS2 expert now for free.