Full Report
The Renesas R7F701381 is a microcontroller for safety critical applications. It contains a second core with a lockstep core (runs instructions in parallel) to check the first core. Additionally, the instruction cache and RAM has ECC as well. Even though the chip has read out protection, the author wants to bypass this on the chip. Renesas chip supports a two wire protocol that can be hooked up with a 5V USB-TTL cable. In order to enable debug mode, a pin needs to be pulled high. However, there was an external watchdog to prevent this was happening. The watchdog would pull the reset line low in order to reset the chip whenever the debug mode was entered. So, the author of the post added a 100 ohm resistor to the 5V line to overpower the watchdog. While in the debug mode, the author tried to connect via the flash programmer for the chip put got an error message from the flasher. Instead of moving on, they decided to analyze the protocol and see if it was the tool deciding it was done or if a custom implementation of the protocol could bypass the protections. The hooked up a logic analyzer and read through documentation of other chips to see what was going on. The documentation of a different chip gave the author a good insight: the command waiting phase is the only location where the device checks if the programmer is allowed. If this stage could be skipped then we would be allowed to use the programmer. Since there was no direct vulnerability in the implementation, the author turned to fault injection. Fault injection is the act of causing the CPU to behave strangely by forcing it outside the normal ranges of operation. For instance, changing the operating voltage, clock frequency or adding in electromagnetic signals. The authors choice was to perform a voltage glitch on the power circuit of the device to modify the course of the program. In particular, they decided to glitch the portion where the synchronize command was being received and checking if debugging was enabled. The datasheet shows a block diagram of the power supply circuit. Inside of the CPU core, there an internal 1.25 voltage regular. To stabilize this, an external pin is attached to a capacitor (VCL). This pin is where the author decided to put the glitch at, since the stability would be affected. Since the RH850 is used for safety critical operations there is a second core being used. As a result, there is a VCL pin for each core! So, the idea was to remove the capacitors for each of these pins to prevent the smoothing out and glitch these cores at the same moment! By doing this, the cores won't go out of sync. The author hooked up an N-FET switch for each of the cores, along with a Raspberry Pi Pico to actually perform the glitching. The Pi would drop the voltage for very small time intervals on the GPIO, which was super controllable. Additionally, the author had reimplemented the protocol with a Python script. Upon each glitch, they would check to see if the debug protocol was returning successfully. After a day of finding the proper parameters, they had a successful debug mode! Each attempt after this only takes 5 to 30 minutes, since we know the parameters now. With this memory can be read out to dump the entire firmware! This was a complete bypass of the functionality in place. Afterwards, they attempted the glitch on ONLY one of the pins. For whatever reason, only glitching the main core works as well, which doesn't make sense to the author. Overall, great post on fault injection which really gets into the interesting details of it.
Analysis Summary
This summary focuses on the techniques and tools used to bypass security features on a Renesas R7F701381 microcontroller, as described in the provided context, rather than traditional malware or threat actor activity.
# Tool/Technique: Voltage Glitching for Debug Access Bypass
## Overview
This entry describes the application of **Fault Injection**, specifically **Voltage Glitching**, targeting the power supply regulation circuit of a Renesas R7F701381 microcontroller. The primary goal was to bypass the integrated read-out protection by forcing the CPU into a state where the debug mode authorization check could be skipped, allowing subsequent firmware dumping.
## Technical Details
- Type: Technique (Hardware Exploitation/Fault Injection)
- Platform: Renesas R7F701381 Microcontroller (Safety-Critical Applications, featuring a dual-core lockstep architecture)
- Capabilities: Bypassing hardware access controls (Read Out Protection) to enable debugging/firmware extraction.
- First Seen: Not applicable (Research/Proof-of-Concept exploitation)
## MITRE ATT&CK Mapping
Since this is a hardware exploitation technique aimed at access and persistence, the mapping focuses on the initial access and defense evasion phases relevant to embedded systems.
- **TA0005** - Defense Evasion
- **T1480** - Exploitation for Privilege Escalation (Hardware mechanism manipulation)
- **TA0001** - Initial Access
- **T1189** - Drive-by Compromise (Indirectly, as physical access is assumed, but the mechanism allows unauthorized access)
## Functionality
### Core Capabilities
1. **Watchdog Countermeasure:** Overpowering an external watchdog timer designed to reset the chip when debug mode entry was attempted, using a 100-ohm resistor on the 5V line.
2. **Protocol Analysis:** Analyzing the proprietary two-wire debug protocol, identifying that the device authorization check only occurs during the "command waiting phase."
### Advanced Features
1. **Targeted Voltage Glitch Injection:** Precisely manipulating the voltage rail (VCL pin) supplying the internal 1.25V regulator for the CPU core(s).
2. **Dual-Core Synchronization Glitching:** Removing external capacitors from both the main core and the lockstep core's VCL inputs to maximize voltage instability simultaneously, ensuring the cores remained synchronized during the fault event.
3. **Hardware Execution Platform:** Utilizing a Raspberry Pi Pico acting as a highly controllable pulse generator (via N-FET switches) to execute the voltage drops at precise moments calibrated against the incoming synchronize command.
4. **Automated Validation:** Employing a custom Python script to re-implement the debug protocol and automatically check for successful debug access after each glitch attempt.
## Indicators of Compromise
- File Hashes: N/A (No specific malware artifacts)
- File Names: N/A
- Registry Keys: N/A
- Network Indicators: N/A (Communication occurs over a proprietary JTAG/SWD-like two-wire protocol)
- Behavioral Indicators: Observing low-level electrical signals on the VCL pins deviating significantly from stable operating voltage during the synchronization phase.
## Associated Threat Actors
- N/A. This represents advanced security research and hardware exploitation methodology, not a known threat actor's tool.
## Detection Methods
- **Signature-based detection:** Not directly applicable to voltage disturbances.
- **Behavioral detection:** Monitoring the hardware for abnormal power fluctuations on critical internal voltage rails (VCL). Monitoring serial/debug ports for unexpected protocol responses or failure to validate proper handshake procedures.
- **YARA rules:** Not applicable.
## Mitigation Strategies
- **Hardware Hardening:** Ensuring robust, non-glitchable power supply circuitry, potentially incorporating voltage monitoring circuits that trigger permanent fuses or secure resets if anomalous dips are detected on critical internal rails.
- **Secure Protocol Implementation:** Implementing authorization checks throughout the entire debug session, not just during the initial command waiting phase.
- **Physical Security:** Implementing tamper-detection mechanisms for external physical access to pins/capacitors, especially those related to internal voltage regulation (like the VCL pins).
## Related Tools/Techniques
- **Fault Injection Frameworks:** Tools like ChipWhisperer (though the implementation used a Raspberry Pi Pico for custom control).
- **Related Techniques:** Clock Glitching, Electromagnetic Injection (LFIA).
- **Bypasses:** Firmware encryption/obfuscation as an additional layer over physical read-out protection.