Pen-testing human-machine interfaces (HMIs) in operational technology environments forces a hard tradeoff: you need to find vulnerabilities, but you cannot afford to stop the process. Legacy systems, incomplete documentation, and protocols like Modbus, DNP3, and OPC UA make that tradeoff harder. The following framework shows how pen-testing HMIs without disrupting live production is achievable—when the methodology is built around operational constraints from the start.
Define Rules of Engagement Before Testing Begins
Every OT cybersecurity assessment starts with a rules of engagement (ROE) document. For HMI testing, this means explicitly naming which systems are in scope—Siemens SIMATIC, Rockwell PanelView, Honeywell Experion—and which are not. The ROE should identify critical and fragile assets, define test windows tied to maintenance schedules, establish escalation contacts within the operations team, and specify exactly which test types are permitted.
Fragile assets deserve special treatment in the ROE. A legacy HMI running on Windows XP may crash under network load that a modern endpoint would handle without issue. Flagging those systems up front, and marking them as passive-only or out of scope entirely, prevents the assessment itself from becoming an incident. Standards like NIST SP 800-82 and IEC 62443 both frame vulnerability testing around documented operational constraints—your ROE is how that framing becomes actionable on the plant floor.
Use Passive Discovery to Map Risk First
Passive discovery should precede any active testing. Analyzing protocol traffic—Modbus TCP, DNP3 over IP, OPC UA—via PCAP captures and flow logs reveals a significant portion of OT risk without touching a single endpoint. Configuration file reviews and structured interviews with OT engineers surface additional exposure: unauthorized remote access points, deprecated protocol usage, HMIs communicating over non-standard ports.
Environments with limited asset inventories or outdated network diagrams benefit most from this approach. A water treatment plant with legacy Allen-Bradley PLCs may have no accurate topology map. Passive analysis builds that map from observed traffic, identifies which HMIs are present, and flags anomalies—all before any active probe is sent. This is the core logic behind a Cyber Vulnerability Risk Assessment (CVRA): identify risk without creating operational risk.
Effective Passive Discovery Techniques
- Network traffic analysis: Inspecting Modbus or DNP3 sessions for anomalous behavior, unexpected source addresses, or unencrypted credential exchange.
- Configuration reviews: Examining HMI configuration files for weak or default passwords, unencrypted communication settings, and unnecessary services.
- Engineering interviews: Engaging OT engineers to surface known vulnerabilities, third-party access arrangements, and undocumented system changes.
Active Testing Must Be Controlled and Rate-Limited
Passive methods alone cannot validate every vulnerability. Active testing is sometimes necessary—but in OT, how you test matters as much as what you test. Active enumeration must be approved in writing through the ROE, rate-limited to avoid flooding industrial network segments, and timed to low-traffic windows or scheduled maintenance periods.
Device sensitivity varies significantly. A Honeywell SM@RT HMI may tolerate limited probing during a planned outage; a critical ABB Ability HMI in a power generation environment may require strict passive-only treatment. The assessment team needs to understand not just the vulnerability surface but the physical consequence of a false positive—network congestion that causes an HMI to miss a process alarm is not an acceptable test outcome. MITRE ATT&CK for ICS provides a useful reference for understanding which technique categories carry the highest operational risk before scoping active test cases.
Standards That Shape Active Test Scope
- IEC 62443: Defines security levels and zone/conduit models that inform which HMI segments can tolerate active testing.
- NIST SP 800-82: Recommends vulnerability testing for HMIs and SCADA systems with explicit attention to availability impact.
- NERC CIP: Requires documented vulnerability assessments for HMIs in scope of bulk electric system coverage.
Combine Automated Scanning With Manual Validation
Automated vulnerability scanners can surface common issues quickly—unpatched HMI software, weak SSH configurations, exposed web interfaces. But automated tools do not understand engineering context, and in OT that context determines whether a finding is a critical risk or an acceptable operational trade-off.
A scanner might flag a Siemens WinCC HMI for running an outdated TLS version. A manual check would determine whether that HMI communicates with a downstream legacy device that cannot support a newer cipher suite—making the finding real but remediable only through a coordinated upgrade, not a simple configuration change. Manual testing can also include controlled scenarios: simulating a credential-stuffing attempt against an HMI login page during a maintenance window, or testing whether a jump server correctly restricts lateral movement toward the process network. Automated tools surface candidates; manual expertise determines what they mean operationally.
Deliver Reports That Drive Operational Decisions
A pen-test report that operations teams cannot act on has not reduced risk. Effective OT assessment reporting includes an executive summary pitched at CISOs and plant managers, technical findings with enough detail to reproduce and validate each vulnerability, and remediation guidance that is prioritized by operational impact—not just CVSS score.
Prioritized remediation matters because not every finding can be patched immediately. A report might recommend replacing a legacy Honeywell HMI during the next planned capital project while implementing a hardened jump server and network segmentation in the near term to reduce exposure in the interim. That sequencing—matching the fix to the operational window—is what distinguishes an OT-aware assessment from a standard IT vulnerability report. Findings should also include risk rationale: why this vulnerability matters in this process environment, not just that it exists.
Pen-Testing HMIs Safely Is a Methodology Problem
The barrier to pen-testing HMIs without disrupting live production is not technical capability—it is methodology. Clear rules of engagement, passive-first discovery, carefully scoped and rate-limited active testing, manual validation with engineering context, and operationally prioritized reporting together form a repeatable approach that identifies real risk without creating new operational exposure. IEC 62443 and NIST SP 800-82 both point toward this kind of structured, risk-aware testing. The discipline is in executing it consistently against environments that were never designed to be tested.
Ready to assess your HMI security posture? Contact Red Trident to discuss a scoped OT cybersecurity assessment built around your operational constraints.
