Sometimes—but not every internet-visible interface is open to anyone, and visibility does not always mean control. In Censys ARC’s October 2024 scan, nearly 400 web-based human-machine interfaces (HMIs) associated with U.S. water facilities were exposed online. Censys classified 40 as unauthenticated and controllable, while others required authentication or were read-only. The findings describe a specific scan, not every water utility or the number exposed today.
What an HMI does at a water or wastewater facility
A human-machine interface is the screen-based control and monitoring interface used by operators to interact with operational technology (OT). In a water facility, an HMI can display information from supervisory control and data acquisition (SCADA) systems connected to programmable logic controllers (PLCs). Operators use it to monitor processes and, depending on the system and permissions, change settings or issue commands.
An HMI reachable over the public internet can disclose operational information such as interface screens, distribution maps, logs, or security settings. If access controls are weak or absent and the interface permits changes, an intruder may be able to alter operations. The specific exposure and risk depend on how the system is configured; an internet-visible HMI is not automatically controllable.
EPA and CISA’s December 13, 2024 fact sheet describes the risks to water and wastewater systems and documents incidents involving manipulated HMIs.
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What the HMI-specific scan found—and what its categories mean
Censys ARC reported nearly 400 web-based HMIs exposed online at U.S. water facilities in its October 2024 scan. Its June 5, 2025 account grouped the identified systems into three access categories:
| Category | Systems in Censys ARC’s October 2024 scan | What the category indicates |
|---|---|---|
| Authenticated | 95 | Authentication was required to access the interface. |
| Read-only | 264 | The scanned interface was classified as viewable but not able to change settings. |
| Unauthenticated | 40 | The interface was classified as accessible without authentication and controllable. |
These distinctions matter. Read-only access can still expose sensitive operational details, while unauthenticated control presents a more direct path to unauthorized changes. Censys does not name the software vendor or model in its account, so the findings should not be attributed to a particular product.
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How the HMI findings differ from EPA OIG’s wider assessment
The figures below describe a separate, broader assessment, not additional HMI counts. The EPA Office of Inspector General’s Report 25-N-0004, published November 13, 2024, says its passive assessment covered 1,062 drinking water systems serving over 193 million people. Its scan date was October 8, 2024.
| EPA OIG finding | Population and risk category | Assessment date and report date |
|---|---|---|
| 97 systems | Serving approximately 26.6 million users; critical- or high-risk vulnerabilities | Scan: October 8, 2024; report: November 13, 2024 |
| 211 systems | Serving over 82.7 million people; medium- or low-risk externally visible open portals | Scan: October 8, 2024; report: November 13, 2024 |
The EPA OIG assessment considered cybersecurity vulnerabilities and externally visible portals broadly, not only HMIs. Its totals should not be added to Censys’s HMI findings or treated as a count of exposed control interfaces. See the EPA OIG report for its scope and findings.
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What is known about remediation—and what is not known today
Censys said EPA and the software manufacturer engaged in remediation after the October 2024 findings. In its May 2025 scan, Censys reported fewer than 20 read-only or unauthenticated systems. That is a dated result from Censys’s scan; the cited sources do not establish how many systems are exposed in October 2026. It should not be presented as a current census.
Why exposed control interfaces can matter operationally
EPA and CISA report that pro-Russia hacktivists manipulated HMIs at water and wastewater facilities in 2024. In documented cases, pump and blower equipment was made to exceed normal operating parameters, and operators had to revert to manual operations. These incidents show that unauthorized HMI access can have operational consequences; they do not mean every exposed interface will cause physical damage.
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How water utilities can reduce HMI exposure
EPA and CISA’s recommended approach is to remove public exposure where feasible, then layer protections around any remote access that must remain. The agencies state: “If possible, disconnect HMIs and all other accessible and unprotected systems from the public-facing internet.”
- Inventory internet-accessible OT. Identify exposed HMIs and other remotely reachable systems so the utility knows what needs protection.
- Remove direct public access where possible. Disconnect HMIs and other accessible, unprotected systems from the public-facing internet when operations allow.
- Secure necessary remote access. Use strong passwords and multifactor authentication (MFA). Place OT behind a demilitarized zone (DMZ) or bastion host rather than exposing the HMI directly. A DMZ can centralize traffic and monitoring; a bastion host provides a secured access point between external and protected networks.
- Limit and monitor connections. Allowlist authorized IP addresses, apply geographic restrictions where appropriate, and log remote logins so access can be reviewed.
- Maintain the system. Apply current patches and follow the product vendor’s security recommendations.
The fact sheet also points water utilities to CISA’s free vulnerability-scanning service. Utilities can consult the EPA/CISA fact sheet for the recommendations and service information.
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