Clock Glitching as an Attack Vector—And How PolarFire® FPGAs Stop It

The Hidden Assumption Behind Secure Systems

Most digital security mechanisms rest on an implicit assumption: time behaves predictably.

Cryptographic engines, authentication checks, secure boot flows and control state machines all depend on precise clock behavior. When clock edges arrive exactly when expected, digital logic behaves deterministically—and securely.

But attackers don’t need to break cryptography or exploit software bugs if they can undermine that assumption. By injecting malformed, shortened or mistimed clock pulses, adversaries can induce faults that cause secure systems to misbehave in subtle yet exploitable ways.

This technique—known as clock glitching—targets the physical execution of logic rather than the correctness of the code itself.

The Problem: Breaking Security by Distorting Clock

Clock glitching attacks operate momentarily violating the timing margins of digital circuits. A single abnormal clock edge can be enough to:

• Skip a critical instruction
• Corrupt a register or comparison result
• Bypass authentication or privilege checks
• Produce incorrect cryptographic outputs

Because the software image remains unchanged, these failures often leave no conventional forensic evidence. To the system, everything appears to have executed “normally”—even though the outcome was compromised.

Why Clock Glitching Is So Dangerous

Digital logic is designed to operate within strict timing margins, and when those margins are violated—even momentarily—the circuitry can enter undefined states. Clock glitching is particularly dangerous because it can be performed externally without modifying firmware, requires no knowledge of internal logic and can be precisely timed to disrupt critical operations such as key comparisons or privilege transitions. Without visibility into clock integrity, a system may never realize a fault injection occurred, leaving it vulnerable to silent compromise.

Real-World Examples

• Secure Boot Bypass Attacks: Researchers have demonstrated clock and voltage glitches that cause bootloaders to skip signature verification, allowing unauthorized firmware to execute.
• Smartcard and MCU Attacks: Carefully timed clock glitches have been used to bypass PIN retry counters and authentication state machines.
• Automotive and Industrial Devices: Fault injection attacks using clock manipulation have extracted firmware and secret material from embedded controllers without invasive probing.

These attacks underscore a critical reality: security checks are only as strong as the timing assumptions they rely on.

The PolarFire^® Family Approach: Hardware-Based Clock Integrity Monitoring

PolarFire FPGAs and PolarFire SoC FPGAs address this threat by integrating clock glitch detection directly into the secure hardware architecture.

Instead of relying on software checks or user implemented timing logic, PolarFire devices implement clock integrity monitoring as a device‑level anti‑tamper feature and route detection events directly into the device’s tamper infrastructure.

The objective is straightforward: detect abnormal clock behavior early and address it before it exploits the system security.

How Clock Glitch Detection Works

Clock glitch detection focuses on validating that the applied clock remains within expected operating parameters.

In operation:

• The device monitors clock characteristics such as cycle duration, frequency stability and pulse integrity
• Incoming clock behavior is compared against internally defined acceptable operating ranges
• Abnormal conditions—such as unexpectedly short or long cycles, missing edges or sudden frequency shifts—are flagged as potential fault-injection events.

When a clock integrity violation is detected, a tamper event is generated by the device:

• In PolarFire FPGA devices, the tamper indication is asserted to the FPGA fabric.
• In PolarFire SoC FPGA devices, the tamper indication is asserted to both the FPGA fabric and the Microprocessor Subsystem (MSS).

Tamper events are routed through the device’s tamper‑response infrastructure, allowing user‑defined mitigation logic to be implemented depending on system‑level security requirements.

By performing detection at the hardware level and signaling violations independently of application execution, PolarFire devices minimize the window of opportunity for clock‑based fault‑injection attacks and give designers fine‑grained control over system response behavior.

Tamper Response: Containing the Attack

Once a clock anomaly is detected by the device’s hardware‑based clock integrity monitors, a tamper event is generated and flagged to the user design.

Based on system requirements, designers can implement configurable countermeasures in response to this tamper indication. These mitigation actions may be implemented in FPGA fabric logic, embedded firmware running on the MSS (on PolarFire SoC devices) or a coordinated combination of both.

 Examples of user‑defined responses include:

• Halting or isolating sensitive logic
• Initiating a controlled or secure system reset
• Zeroizing cryptographic keys or sensitive data stored in secure memory
• Disabling external interfaces or debug access
• Transitioning the system into a failure-safe state

Clock anomaly detection itself is fully implemented in dedicated hardware and operates independently of the user design. When a violation is detected, the hardware asserts a tamper indication and provides it to the user design, which then executes the selected response actions based on the system’s security policy.

This model allows fault‑injection attempts to be detected at the hardware level while still giving system designers full flexibility to define how the system responds.

Use Case: Protecting Secure Boot and Authentication

Secure boot, authentication and other security‑critical transitions are prime targets for clock‑glitching attacks. A precisely timed anomaly during signature verification, secure‑to‑non‑secure transitions or other privilege changes can cause a system to accept unauthorized code or enter an unintended execution state.

By linking clock glitch detection to tamper response logic, PolarFire based designs can create confidence that:

• Abnormal timing behavior during secure boot and authentication is detected early
• Faults during transitions between secure and non‑secure execution contexts are identified
• The system fails safely rather than continuing operation in an undefined or insecure state

Clock integrity monitoring is also valuable during cryptographic key operations, such as key generation or exchange, where timing‑based fault injection may otherwise disrupt cryptographic correctness or expose sensitive material.

In these scenarios, clock integrity monitoring complements cryptography and secure boot by helping detect and contain timing‑based attacks that target control flow rather than algorithms.

Conclusion: Security Depends on Trustworthy Clocks

As fault‑injection attacks move beyond software vulnerabilities and target timing behavior directly, preserving clock integrity becomes a critical aspect of system security.

Clock glitch detection in PolarFire FPGAs and PolarFire SoC FPGAs strengthens security by helping prevent timing‑based attacks from undermining otherwise robust protection mechanisms.