Legacy Operational Technology (OT) and Industrial Control System (ICS) infrastructure continues to create major security challenges for industrial organisations. Many ageing systems were never designed for today’s connected environments, yet they still support critical operations across manufacturing, utilities, transport and energy.

Below are 11 hidden OT attack paths commonly created by ageing environments and the practical controls organisations can implement to reduce risk quickly.

1. Unsupported operating systems connected to production environments

Many industrial environments still rely on unsupported operating systems that no longer receive security updates. These systems often remain deeply embedded within production processes because replacing them would require expensive operational shutdowns or revalidation of critical systems.

Threat actors actively target these assets because vulnerabilities are well documented and exploit tools are widely available. Once compromised, these systems can provide attackers with a stable foothold inside operational environments.

Industrial organisations should prioritise network isolation for unsupported systems, implement strict segmentation policies and use virtual patching where direct updates are not operationally possible. Continuous monitoring of traffic to and from legacy assets can also help identify suspicious activity early.

2. Unsecured remote vendor access

Third-party remote access has become one of the most common intrusion points in OT environments. Vendors often require access for diagnostics, maintenance and support, but older environments frequently rely on persistent VPN connections, shared credentials or poorly monitored remote desktop services.

Attackers increasingly target suppliers and contractors because they often provide trusted access into critical environments while bypassing standard security controls.

Industrial organisations should implement privileged access management, enforce MFA across all remote connections and use time-limited access sessions with full activity logging. Remote access pathways should also be segmented away from critical operational systems wherever possible.

3. Forgotten serial-to-Ethernet converters

Many legacy industrial control systems were originally built around serial communications before Ethernet became common across operational environments. As organisations modernised, serial-to-Ethernet converters were introduced to bridge old and new infrastructure.

Over time, many of these devices were forgotten or excluded from formal security inventories. Because they often sit quietly within the environment, they can create hidden attack paths directly into industrial controllers and field devices.

Comprehensive asset discovery and OT network mapping are critical for identifying legacy communication bridges. Once identified, these devices should be isolated into tightly controlled network zones and monitored for abnormal traffic behaviour.

4. Flat OT network architecture

Many ageing OT environments still operate with minimal internal segmentation. Historically, these networks were designed around reliability and operational simplicity rather than cybersecurity resilience.

The result is that once attackers gain access to a single device, lateral movement across the operational environment can become relatively straightforward. This significantly increases attack surface and threat exposure in OT environments.

Strong segmentation remains one of the most effective ways to reduce operational risk. Separating OT from IT environments, isolating critical systems and restricting east-west traffic can dramatically reduce the ability of attackers to move through industrial networks.

5. Legacy engineering workstations

Engineering workstations often represent some of the most valuable systems within industrial environments. They typically contain elevated privileges, direct PLC access and sensitive operational configurations.

Unfortunately, these systems are frequently excluded from regular maintenance cycles because downtime concerns limit patching opportunities. Some continue operating with outdated antivirus software, unrestricted USB access and minimal monitoring.

Application allowlisting, endpoint monitoring and restricted removable media policies should be prioritised for engineering systems. Organisations should also minimise internet connectivity from these workstations wherever operationally feasible.

6. Unmanaged removable media

USB devices remain widely used throughout industrial environments for diagnostics, updates and transferring operational files. However, removable media continues to represent one of the easiest malware delivery mechanisms into OT environments.

Many older environments still lack consistent removable media governance, scanning controls or transfer procedures between IT and OT systems.

Dedicated USB sanitisation stations, authorised device policies and strict transfer procedures can significantly reduce the risk of malware entering production environments through removable media.

7. Internet-exposed HMI systems

Human Machine Interface systems occasionally become exposed to the internet during remote troubleshooting, temporary vendor access arrangements or poorly managed operational changes.

Threat actors actively scan for exposed HMI systems because they can provide direct visibility into industrial operations and, in some cases, the ability to manipulate processes.

External attack surface monitoring and regular exposure assessments are essential for identifying internet-facing OT assets before attackers discover them.

8. Weak default credentials in embedded devices

Many industrial devices continue operating for years using default manufacturer credentials because changing them may impact vendor support agreements or disrupt legacy integrations.

These credentials are often publicly available online and are frequently used during attacks targeting industrial control systems vulnerabilities.

Industrial organisations should implement structured credential management processes, rotate privileged accounts regularly and continuously monitor authentication activity across OT environments.

9. Unpatched PLC firmware

Asset lifecycle management and patching remains one of the biggest challenges in operational technology cybersecurity. Many PLCs and controllers operate continuously with limited maintenance windows, making firmware updates difficult to schedule safely.

As a result, industrial organisations often continue operating systems with known exploitable vulnerabilities for extended periods.

Rather than attempting broad patching programmes across all assets, organisations should prioritise remediation efforts based on exploitability, operational criticality and exposure pathways.

10. Insecure industrial protocols

Protocols such as Modbus, DNP3 and older OPC implementations were never designed with modern cybersecurity requirements in mind. Most lack encryption, authentication and integrity controls entirely.

Attackers able to access these communication channels may intercept commands, manipulate industrial traffic or disrupt operational processes without detection.

Industrial intrusion detection systems, deep packet inspection and OT-aware network monitoring can improve visibility into suspicious protocol activity and abnormal communications.

11. Shadow OT assets outside governance

Many industrial environments contain undocumented systems, temporary deployments and unmanaged assets that sit outside formal governance processes. These shadow assets frequently evade vulnerability management, monitoring and security reviews entirely.

Without complete visibility, organisations cannot accurately assess industrial control systems vulnerabilities or understand the true extent of legacy systems risk management challenges.

Continuous asset discovery and centralised OT visibility are essential for identifying unmanaged systems before attackers exploit them.

How Integrity360 can help

Securing operational technology environments requires specialist expertise that understands both industrial operations and modern cybersecurity threats. Integrity360’s OT Security Services help organisations identify hidden exposures, strengthen segmentation, improve visibility and reduce operational risk across complex OT and ICS environments.

From Industrial 360 Audits and OT penetration testing to OT incident response, engineering support and ransomware resilience assessments, Integrity360 provides tailored services designed specifically for industrial operations.

With more than 40 OT security consultants, over 250 OT penetration tests conducted annually and experience supporting organisations across five continents, Integrity360 helps industrial organisations uncover hidden attack paths before attackers do.

If your organisation is concerned about outdated industrial control system assets and OT security risk, now is the time to act.

Contact Integrity360 to assess your OT environment, identify critical exposures and strengthen the resilience of your operations.