Protecting the Core: Securing Protection Relays in Modern Substations

Network Topologies: Substations employ redundant Ethernet designs to achieve high availability and zero-packet-loss communication, especially for protection-critical traffic.

Common topologies include:

• RSTP (Rapid Spanning Tree Protocol) – Basic redundancy by blocking loops in switched networks

• PRP (Parallel Redundancy Protocol) – Simultaneous frame delivery over two independent paths

• HSR (High-availability Seamless Redundancy) – Ring-based protocol that allows seamless failover for protection traffic

Communication Layers: Zones and Roles

Modern substations are structured into distinct functional network layers, each responsible for different operations, timing profiles, and security domains. Understanding this layered architecture is critical to both operational design and cyber risk modeling.

What Protection Relays Really Do

In modern digital substations, protection relays—more accurately referred to as IEDs—have evolved far beyond basic trip-and-alarm functions. These devices now serve as cyber-physical control points, responsible not only for detecting faults in real time but also for executing programmable logic, recording event data, and acting as communication intermediaries between digital and legacy systems.

At their core, IEDs monitor electrical parameters, such as voltage, current, frequency, and phase angle, and respond to conditions like overcurrent, ground faults, and frequency deviations. Upon fault detection, they issue trip commands to circuit breakers—typically within one power cycle (e.g., 4–20 ms)—to safely isolate the affected zone and prevent equipment damage or cascading outages.

• Beyond traditional protection: Modern IEDs provide a rich set of capabilities that make them indispensable in fully digitized substations.

  • Trip Logic Processing: Integrated logic engines (e.g., SELogic, FlexLogic, CFC) evaluate multiple real-time conditions to determine if, when, and how to trip, block, or permit operations.

  • Event Recording and Fault Forensics: Devices maintain Sequence of Events (SER) logs and capture high-resolution oscillography (waveform data), supporting post-event diagnostics and root-cause analysis.

  • Local Automation Capabilities: IEDs can autonomously execute transfer schemes, reclose sequences, interlocking, and alarm signaling often without intervention from SCADA or a centralized controller.

  • Protocol Bridging and Communication Integration: Most modern relays support and translate between multiple protocols, including IEC 61850, DNP3, Modbus, and IEC 60870-5-104, enabling them to function as data gateways or edge translators in hybrid communication environments.

• Application across the grid: These devices ensure rapid fault isolation, coordinated protection, and reliable operation across transmission, distribution, and industrial networks.

  • Transmission and distribution lines (e.g., SIPROTEC)

  • Power Transformers (e.g., ABB RET615)

  • Feeders, Motors and industrial loads (e.g., GE D60)

How Attackers Can Recon and Target Protection Relays

As substations evolve into digital control hubs, their critical components, particularly protection relays, are no longer isolated devices. These IEDs are now network-connected through Ethernet, serial-to-IP converters, USB interfaces, and in rare cases, tightly controlled wireless links used for diagnostics or field tools.

While this connectivity improves maintainability, remote engineering access, and real-time visibility, it also expands the cyberattack surface exposing relays to risks of unauthorized logic modification, protocol exploitation, or lateral movement from compromised engineering assets.

Reconnaissance From the Internet

Attackers often begin with open-source intelligence (OSINT), building a map of the organization’s digital and operational footprint. They aren’t initially looking for IEDs or substations; they’re identifying the humans who manage them.

• Social Recon: Using LinkedIn, engineering forums, or vendor webinars, attackers look for job titles like “Substation Automation Engineer,” “Relay Protection Specialist,” or “SCADA Administrator.”

• OSINT Targeting: Public resumes and RFI documents may reference software like DIGSI, PCM600, or AcSELerator. Even PDF metadata from utility engineering documents can reveal usernames, workstation names, or VPN domains.

• Infrastructure Scanning: Tools like Shodan or Censys help identify exposed VPNs, engineering portals, and remote access gateways. If these systems support weak authentication or use outdated firmware, they become initial entry points.

• Exploitation of Weak Vendor Access: Many utilities still use stand-alone VPN credentials for contractors and OEM vendors. These accounts often bypass centralized identity systems, lack 2FA, and are reused across projects.

Reconnaissance in IT — Mapping the Path to OT

Once an attacker gains a foothold within the IT network—typically through phishing, credential theft, or exploiting externally exposed services—their next objective shifts toward internal reconnaissance. The target is not just domain dominance, but lateral movement toward OT-connected assets such as substations or Energy Management Systems (EMS).

• Domain Enumeration: Using tools like BloodHound, attackers map Active Directory for accounts, shares, and systems tagged with OT context (e.g., usernames like scada_substation_admin, and groups like scada_project and scada_communication).

This phase allows the attacker to pinpoint high-value users and their associated devices, building a shortlist of engineering staff, contractors, or control center personnel who likely interface with OT assets.

• Workstation & Server Access: Armed with domain privileges and OT-centric intelligence, the attacker pivots to target the workstations or terminal servers used by the identified engineers. These endpoints are rich in substation-relevant data, such as:

  • Relay configuration files (.cfg, .prj, .set)

  • VPN credentials or profiles for IDMZ access

  • Passwords embedded in automation scripts or connection managers

  • Access logs or RDP histories indicating commonly used jump hosts

At this stage, the attacker is no longer scanning blindly; they’re executing highly contextual moves to identify paths from IT into OT.

IDMZ Penetration — Crossing the Last Boundary

Using gathered VPN credentials, hard-coded SSH keys, or jump host details, the attacker attempts to cross into the DMZ. This zone typically mediates communication between IT and OT, and may be accessed via:

• Engineering jump hosts (dual-homed systems, often less monitored)

• Poorly segmented RDP gateways with reused credentials

• Exposed management ports on firewalls or remote access servers

Once in the IDMZ, attackers map accessible subnets and identify potential pathways into live substations.

Substation Discovery and Technical Enumeration

Once an attacker successfully pivots into the substation network often via compromised VPN credentials, engineering jump hosts, or dual-homed assets bridging corporate and OT domains—the next step is to quietly enumerate the substation landscape. At this point, they are no longer scanning broadly but conducting targeted reconnaissance to identify and isolate high-value assets, particularly protection relays.

Rather than using noisy tools like nmap with full port sweeps, attackers rely on stealthier techniques tailored for industrial networks. These include passive traffic sniffing and protocol-specific probing to avoid triggering intrusion detection systems or log correlation engines. For example, using custom Python or Scapy scripts, the attacker might issue minimal handshake packets for protocols such as IEC 61850 MMS, DNP3, or Modbus, observing how devices respond to crafted requests. This helps fingerprint device types and capabilities without sending bulk probes.

Simultaneously, MAC address analysis plays a crucial role in identifying vendors. Many industrial devices use identifiable prefixes unique to specific power control system manufacturers. Attackers often leverage this to differentiate protection relays from HMIs, RTUs, or gateways with a high degree of accuracy.

Additionally, by observing mirrored traffic on span ports or through passive sniffing on switch trunks, attackers can detect GOOSE messages, Sampled Values (SV), or heartbeat signals indicative of live relay communication. These traffic patterns confirm the presence of active IEDs, and in some cases, help infer the device’s operational role or logical zone.

Once relays, protocol gateways, and engineering HMIs have been identified, the attacker begins deeper technical enumeration. At this stage, they analyze which services are exposed on each device such as Telnet, HTTP, FTP or MMS, and gather banner information or port responses that reveal firmware versions, relay models or serial numbers. Devices with weak authentication or legacy configurations are prioritized for exploitation.

The attacker may next attempt to log in using factory-set or default credentials, which are often easily obtainable from device manuals. Alarmingly, these credentials are often still active in many substations due to lax commissioning processes. If login is successful, the attacker escalates from passive enumeration to active control—gaining the ability to view or modify protection settings, trip logic, and relay event logs.

If the relays are hardened with proper credentials or access controls, attackers might try other methods, such as accessing rear-panel serial ports via local connections or probing serial-over-IP bridges linked to terminal servers. Some adversaries have even used vendor software (e.g., DIGSI, AcSELerator, PCM600) found on compromised engineering workstations to open relay configuration projects, review programmable logic (e.g., SELogic or FlexLogic), and make changes through trusted interfaces.

Another critical risk in substation environments is the presence of undocumented or hidden device functionality. As highlighted in CISA advisory ICSA-24-095-02, SEL 700-series protection relays were found to contain undocumented capabilities accessible to privileged users.

Separately, some relays may expose backdoor Telnet access through hard-coded or vendor diagnostic accounts. These interfaces are often enabled by default and left undocumented, giving attackers an opportunity to inject firmware, wipe configurations, or issue commands that can directly trip or disable breakers.

By the end of this quiet but highly effective reconnaissance phase, the attacker has mapped out the protection relay landscape, assessed device exposure, and identified access paths. They now shift from understanding the network to understanding what each relay actually controls, entering the next phase: process-aware enumeration.

Process-Aware Enumeration

Once attackers have quietly mapped out the substation network (identifying protection relays, protocol gateways, engineering HMIs, and confirming which devices expose insecure services) their focus shifts from surface-level reconnaissance to gaining operational context. Discovery alone isn’t enough. For any compromise to deliver strategic impact, adversaries must understand how these devices interact with the physical power system.

This is where process-aware enumeration begins. The attacker is no longer interested in just controlling any relay they want to control the right relay. That means understanding what each device protects, how it’s wired into the breaker scheme, and what its role is within the substation topology.

Armed with access to engineering workstations or backup file shares, the attacker reviews substation single-line diagrams (SLDs), often from SCADA HMI screens or documentation from project folders. These diagrams reveal the electrical architecture—transformers, feeders, busbars—and show exactly where each relay fits. Identifiers like “BUS-TIE PROT” or “LINE A1 RELAY” are matched against configuration files to determine their protection zone.

By correlating relay names with breaker control logic and protection settings, the attacker maps out zone hierarchies: primary and backup relays, redundancy groups, and dependencies between devices. They identify which relays are linked to auto-reclose logic, which ones have synch-check interlocks, and which outputs are shared across multiple feeders.

This insight enables precise targeting. For example, instead of blindly disabling protection across the board, which would raise immediate alarms, the attacker may suppress tripping on a backup relay while leaving the primary untouched. Or, they might modify logic in such a way that a fault won’t be cleared until the disturbance propagates, creating the conditions for a wider outage.

At this stage, the attacker is not just exploiting the relay as a networked device. They’re treating it as a control surface for the substation itself. With deep process context in hand, they move from reconnaissance to exploitation: manipulating logic, altering protection thresholds, injecting malicious firmware, or spoofing breaker commands and because their changes are aligned with system topology, they maximize impact while minimizing detection.

Practical Examples of Exploiting Protection Relays

The fusion of network awareness and electrical process understanding makes modern substation attacks particularly dangerous—and why protection relays, when compromised, represent one of the highest-value cyber-physical targets in the grid.

To illustrate how such knowledge is operationalized by attackers, let’s examine a practical example involving the SEL-311C relay, a device widely deployed across substations. Note: While this example focuses on SEL, the tactics described here apply broadly to comparable relays from other major OEM vendors such as ABB, GE, Siemens, and Schneider Electric. In addition, the information presented in this section does not constitute any unknown vulnerabilities or proprietary information, but instead demonstrates the potential for an attacker to use built-in device features to achieve adversarial objectives.