How to Prioritize Patches for PLCs, RTUs, and HMIs in Industrial Cybersecurity

The Importance of Patch Management in Industrial Cybersecurity

In today’s highly connected world, the cybersecurity of Operational Technology (OT) is a critical concern, especially when it comes to programmable logic controllers (PLCs), remote terminal units (RTUs), and human-machine interfaces (HMIs). These devices form the backbone of industrial processes and manufacturing systems, making them prime targets for cyberattacks. While most IT systems undergo regular patching to protect against vulnerabilities, OT systems are often overlooked or delayed due to operational constraints. However, with the increasing convergence of IT and OT, a strategic approach to patch management is no longer optional – it’s essential.

This blog post will provide a comprehensive guide on how to prioritize patches for PLCs, RTUs, and HMIs, ensuring that organizations can reduce risk without disrupting their critical industrial operations. We’ll delve into the process of identifying vulnerabilities, assessing risks, and effectively implementing patching procedures, tailored specifically to the unique demands of OT environments.

Why Prioritize Patching for OT Systems?

Before diving into the specific steps for prioritizing patches, it’s important to understand the unique security challenges that OT systems face compared to traditional IT systems. OT systems, which control physical processes, are often built with long lifecycles (sometimes spanning decades). These systems are typically more vulnerable due to several reasons:

1. Legacy Systems: Many OT devices were not designed with cybersecurity in mind. As a result, they often lack the ability to patch vulnerabilities in a timely manner.
2. Operational Continuity: Unlike IT environments, where updates can be performed without major disruptions, patching in OT systems can lead to downtime, which directly impacts production and safety.
3. Isolation vs. Connectivity: Historically, OT systems were isolated from the outside world for security reasons. However, with increased connectivity for data sharing and cloud integration, these systems are now more exposed to cyber threats.

Due to these challenges, effective patch prioritization is crucial to strike a balance between maintaining operational uptime and securing critical systems. A poor patching strategy can lead to catastrophic vulnerabilities being exploited, while excessive patching without due consideration may result in unnecessary operational disruption.

Step 1: Identify and Categorize Devices

The first step in any patch management strategy is identifying all devices within the OT environment. This includes not only PLCs, RTUs, and HMIs but also any ancillary devices such as sensors, actuators, and field devices.

PLCs (Programmable Logic Controllers): PLCs are essential for controlling machinery in industrial environments. They monitor inputs and control outputs in real-time. A vulnerability in a PLC could allow attackers to manipulate critical machinery, leading to dangerous outcomes.

RTUs (Remote Terminal Units): RTUs are used to collect data from sensors and transmit it back to a central monitoring system. These devices are often deployed in remote locations and play a key role in supervisory control and data acquisition (SCADA) systems. If an RTU is compromised, an attacker could disrupt data collection or even control critical systems remotely.

HMIs (Human-Machine Interfaces): HMIs allow operators to interact with and monitor industrial processes. Compromising an HMI can provide attackers with control over the entire system, including the ability to adjust parameters or disable safety measures.

Once these devices are identified, categorizing them based on their role and risk is essential. Critical devices, such as those controlling safety mechanisms or sensitive processes, should be prioritized for patching.

Step 2: Assess and Prioritize Vulnerabilities

Once all devices are categorized, the next step is to assess the vulnerabilities that each device faces. This is done by referencing publicly available vulnerability databases (such as the National Vulnerability Database, NVD) and advisories from the device manufacturers or cybersecurity organizations.

Key factors to consider when assessing vulnerabilities:

1. Severity of the Vulnerability: Use a standardized vulnerability scoring system, such as the Common Vulnerability Scoring System (CVSS), to gauge the potential impact of the vulnerability. Focus on vulnerabilities with a high CVSS score, especially those with potential for remote exploitation.
2. Exploitability: Determine how easy it would be for an attacker to exploit the vulnerability. Some vulnerabilities may require extensive resources or access to the network, while others may be easily exploitable.
3. Potential Impact: Consider the possible consequences of a successful attack. For example, a vulnerability in a safety-critical PLC could have far-reaching consequences, whereas an issue with a less critical RTU may not have the same impact.
4. Exposure: Assess the level of exposure of each device. Devices that are directly accessible from external networks or poorly secured are higher risk and should be patched sooner.

Step 3: Create a Patch Management Schedule

With the vulnerabilities identified and prioritized, the next step is to create a patch management schedule. This schedule should account for both the urgency of the patches and the operational constraints of the environment.

Best practices for scheduling patch deployments:

1. Patch in Phases: Rather than applying patches all at once, stagger deployments over time to minimize disruption. Start with non-critical systems or test environments to ensure compatibility before rolling out to critical devices.
2. After-Hours Patching: If possible, schedule patching during non-production hours to reduce downtime. However, ensure that any necessary failover systems are in place to prevent interruptions to production during patching.
3. Test Patches: Always test patches in a controlled environment before deploying them to production systems. This ensures that the patch does not negatively affect the device or cause instability.
4. Backup Systems: Before applying any patch, ensure that all critical systems are backed up, and rollback plans are in place in case something goes wrong during the update process.

Step 4: Implement a Robust Monitoring Strategy

Patching is only effective if vulnerabilities are continuously monitored and remediated in a timely manner. To ensure ongoing protection:

1. Regular Vulnerability Scanning: Continuously scan the OT network for new vulnerabilities using automated tools. This helps ensure that patches are up to date and that new risks are identified and mitigated promptly.
2. Integrate with IT Systems: As OT and IT systems become more integrated, ensure that patch management tools for IT (such as WSUS or SCCM) can also manage OT patches. This integration allows for streamlined patching across the entire infrastructure.
3. Continuous Monitoring: Implement real-time monitoring to detect any signs of exploitation or cyberattacks. Intrusion detection systems (IDS) and security information and event management (SIEM) systems can provide critical insights into any abnormal activity following a patch deployment.

Step 5: Document and Report

Finally, it’s essential to document all patching activities, including the reasons for patch prioritization, testing results, and successful deployments. Documentation not only supports compliance with regulatory standards but also ensures that teams can track patching efforts over time.

Key elements to document:

1. Patch Summary: Include details about each patch applied, including the severity of the vulnerability it addressed and the devices affected.
2. Patch Test Results: Document test results to confirm that the patch did not cause any disruptions or negative impacts.
3. Post-Patching Status: Ensure that the patch was successfully deployed and that all systems are functioning correctly.
4. Compliance Reporting: For industries with strict regulatory standards (such as healthcare or critical infrastructure), detailed patch management documentation may be required for compliance purposes.

Conclusion: Balancing Security and Operational Continuity

The process of patching PLCs, RTUs, and HMIs is vital for protecting OT environments from cyber threats, but it must be done strategically. By understanding the risks associated with each device, assessing vulnerabilities, scheduling patches appropriately, and continuously monitoring for new threats, organizations can significantly reduce the risk of cyberattacks while ensuring operational continuity.

The rise of IT-OT convergence means that effective patch management is more critical than ever. With the right tools and strategies in place, organizations can maintain a robust security posture while minimizing disruptions to production processes.