How to Protect PLCs and HMIs from Cyber Threats: A Complete Guide for Food Manufacturers

In many food manufacturing plants, legacy systems like Siemens S7 PLCs, older HMIs, and outdated Windows-based control computers remain at the core of production lines. While these systems keep operations running, they also introduce critical cybersecurity risks. A cyberattack targeting these devices can halt production, spoil products, and even create safety hazards.

This guide explains how to protect PLCs and HMIs from cyber threats with actionable strategies for food manufacturers to secure legacy industrial systems and maintain uninterrupted, safe production.

1. Assess the Situation Immediately

When facing cybersecurity threats or system irregularities, rapid assessment is essential.

• Identify vulnerable systems: Determine which PLCs, HMIs, or SCADA devices are exposed. Legacy systems like Windows XP HMIs or Siemens S7-300/400 PLCs are particularly at risk.
  
• Evaluate severity: Determine if automated processes are at a complete stop or if operations can continue manually.
  
• Consider safety risks: Perishable foods, refrigeration, and critical control points may be compromised.
  

Tip: Prioritize devices whose compromise could affect food safety, spoil products, or cause production halts. In food manufacturing, cybersecurity downtime is not just operational; it can threaten consumer safety.

2. Protect Food Safety During System Threats

While technical measures are applied, it is critical to safeguard products.

• Temperature control: Ensure cold foods stay between 32–41°F and frozen products remain at 0°F or below. Minimize time in the danger zone, which is 40–140°F.
  
• Prevent cross-contamination: Isolate batches affected by system failures and follow Good Manufacturing Practices (GMP).
  
• Sanitation checks: Enforce strict hygiene and protective protocols for staff handling products during incidents.
  

3. Activate Your Incident Response Plan

Every facility should have a formal plan for system threats.

• Tiered response: Quickly escalate threats to IT and OT experts.
  
• On-site spare parts: Maintain critical PLC modules, sensors, and network switches for rapid replacement.
  
• Remote monitoring and troubleshooting: Use OT-friendly IDS and IPS tools as well as remote access tools to diagnose HMI and SCADA issues. Coordinate with on-site staff for hands-on remediation.
  

Pro tip: Threats often cascade beyond one device. Monitor supporting controllers, interlocks, and networks to prevent wider disruptions.

4. Coordinate With Your Team

Clear communication and accountability minimize risk during attacks.

• Assign responsibilities: Designate staff for system restoration, food safety oversight, and regulatory documentation.
  
• Document actions: Record affected systems, actions taken, and product status for compliance and incident analysis.
  
• Maintain alignment: Ensure operators, maintenance, and management are synchronized to prevent confusion during response.
  

5. Conduct Technical Troubleshooting

Once immediate risks are mitigated, begin securing the automation systems.

• Inspect and reboot critical systems: Check PLCs, HMIs, and SCADA. Ensure firmware versions are correct.
  
• Verify network security: Segment networks, check VLANs, industrial firewalls, and verify router and switch configurations.
  
• Monitor for anomalies: Use diagnostic tools like S7-Online and ICS-friendly IDS and IPS platforms such as Nozomi Networks or Claroty to detect unauthorized commands or firmware changes.
  

6. Resume Production Safely

After systems are secured:

• Controlled restart: Start with a small batch to confirm systems and safety mechanisms function correctly.
  
• Monitor critical points: Temperature sensors, flow meters, and alarms should be verified before full-scale production.
  
• Inspect affected products: Discard or safely reprocess items exposed to unsafe conditions.
  

7. Prevent Future Cybersecurity Incidents

The best defense is proactive preparation.

• Network segmentation and isolation: Keep PLCs and HMIs in dedicated OT networks that are isolated from IT and the internet. Use DMZs and firewalls for controlled communication.
  
• Virtual patching and industrial proxies: Apply network-level filters for legacy devices that cannot be upgraded. Inspect protocols like S7Comm for compliance and reject malicious traffic.
  
• Application whitelisting: Limit execution on Windows HMIs to approved software paths only, for example, C:\Program Files\Siemens\WinCC\*.exe.
  
• Maintain spares and documentation: Track firmware versions, device diagrams, and critical spares for quick recovery.
  
• Operator training and policies: Educate staff on phishing, USB hygiene, and device policies. Formalize legacy device management and decommissioning plans.
  
• Regular monitoring: Deploy IDS and IPS tools, log device activity, and review anomalies to detect early threats.
  

8. Understand the Financial and Safety Impacts

Legacy PLCs and HMIs are a critical vulnerability.

• Lost revenue: Production stoppages can cost hundreds of thousands of dollars per hour.
  
• Spoiled inventory: Temperature breaches may render products unsafe.
  
• Regulatory penalties: Noncompliance with food safety regulations can result in fines or plant shutdowns.
  
• Emergency repair costs: Rapid remediation is expensive without preparation.
  

Mitigation strategies reduce these risks by ensuring systems remain operational, safe, and monitored even under threat.

Securing Legacy PLCs and HMIs

Protecting legacy PLCs and HMIs in food manufacturing requires a multi-layered cybersecurity approach. While modernization is the long-term solution, immediate priorities include isolating critical devices, monitoring activity, applying virtual patching, and training operators.

By following these steps, food manufacturers can reduce downtime, safeguard consumer safety, and maintain uninterrupted production even when using older automation systems.