Full Report
The author of this post had recently bought a Phillips Sonicare toothbrush. When reviewing the documentation, it says that the product operates at 13.56MHz, which indicates this uses NFC. The communicate happens from the toothbrush handle to the toothbrush head to tell you that a new one is needed. Using the NFC tools app, there is a lot we can learn about the tag. Some interesting notes from this: The tag is NTAG213 and uses NfcA. The device is password protected. Address 0x24 contains the total brush time. Various other fields such as checksums, IDs, links and more. The goal of the attack is to overwrite the brushed time of the device. Reversing engineering the format is easy: observe the value, brush your teeth and observe again. However, it's password protected. So, what do we do? Software Defined Radio (SDR) time! The author pulled out a HackRF (which goes down to 1MHz for listening). Then, they used Gnu Radio to take the raw IQ signals and convert the data into a WAV file. The WAV file is composed of a seris of complex numbers, which is why the author pulled out the real and imaginary portions to convert it to a WAV. The tool NFC-laboratory takes in a WAV file and decoding the bytes manually for us. By cross-referencing the data being sent and the stored data, we can learn what's being sent from the WAV file. Lines 0-6: Establish communication between the systems. Lines 7: Send password - 0x1B command. Lines 9: The counter is updated to a new value - 0xA2 command. Reading line 7 shows that the password is 67:B3:8B:98! Woah, that's awesome. It's super crazy to me that the data is sent in plaintext over NFC; I figured some type of encryption would be done by default. Using NFC tools, we can set the password of the field then attempt to edit the field like we tried before. Since the password is set, it works! We've got a NEW toothbrush as far as the device thinks. Through the post, the password changes. What gives? Attempting the password incorrectly 3 times will permanently disable write access. Additionally, the passwords are unique per toothbrush. NXP recommends that the password should be generated from the UID but the author couldn't find the transformation function. Overall, an amazing post! I learned all about WAV files and NFC hacking.
Analysis Summary
# Tool/Technique: NFC Hacking via SDR and Gnu Radio
## Overview
This describes a set of techniques used to reverse engineer, bypass security, and manipulate data stored on an NTAG213 NFC tag embedded within a Philips Sonicare toothbrush head, specifically targeting the total brush time counter. The process involved using a Software Defined Radio (SDR) to capture, decode, and analyze the unencrypted radio frequency communication between the toothbrush handle and the brush head.
## Technical Details
- Type: Tool / Technique
- Platform: NFC Tags (NTAG213), Hardware (HackRF, Philips Sonicare)
- Capabilities: Capturing raw RF signals, decoding NFC communication, extracting plaintext passwords, manipulating password-protected memory fields.
- First Seen: Not explicitly stated, but the disclosure of the exploit occurred around May 2023 (based on linked articles).
## MITRE ATT&CK Mapping
This activity primarily falls under Reconnaissance, Collection, and Defense Evasion, specifically concerning hardware interfaces.
- **TA0043 - Web Application** (Applicable contextually as it manipulates a device interface)
- **T1551.002 - Software Discovery: Cloud Service** (Least relevant)
- *Note: Direct hardware/IoT mappings are often covered under specialized ATT&CK extensions, but for core frameworks, collection and discovery are the closest fits.*
- **TA0007 - Discovery**
- **T1598.003 - Search Engine: Targeted Search Engine Queries** (Analogous to targeted reverse engineering)
- **TA0005 - Defense Evasion** (Related to overriding device logic)
- **T1562.006 - Disable or Modify Tools: Hardware Defenses** (Modifying the lock status via password)
- **TA0009 - Collection**
- **T1560.001 - Archive Collected Data: Archive via Utility** (Capturing IQ signals)
*(Self-Correction based on techniques used: The core technique is **Hardware Interface Discovery and Manipulation** rather than standard software/network TTPs, but the provided framework maps to more universally recognized entries. The derivation of the password via sniffing aligns best with **Adversary-in-the-Middle / Protocol Analysis** which often maps to Collection/Discovery.)*
## Functionality
### Core Capabilities
- **NFC Protocol Analysis:** Utilizing NFC Tools to identify tag type (NTAG213, NfcA), memory map, and security features (password protection).
- **Brush Time Measurement:** Reversing the format of stored data at address `0x24` which tracks total brush time in seconds (with potential rollover).
- **RF Signal Capture:** Using a HackRF SDR to tune to the operating frequency (13.56 MHz) and record raw IQ signals during communication bursts using Gnu Radio.
- **Data Conversion:** Converting raw IQ recordings (complex numbers) into a discernible signal format (WAV file) for manual byte inspection.
- **Plaintext Password Extraction:** Identifying specific commands (e.g., `0x1B` for password send) within the captured WAV data to extract the 4-byte password in plaintext.
### Advanced Features
- **Password Bypass/Manipulation:** Once the password was known, standard NFC tools were used, with the added password context, to change memory contents, effectively resetting the brush head usage counter.
- **Permanent Write Lock Observation:** Understanding the NXP NTAG security feature where three incorrect password attempts permanently disable all write access (Configuration Lock bit changes).
- **UID Correlation Hypothesis:** Observing that passwords were unique per tag and hypothesized to be generated based on the tag's UID, as suggested by NXP documentation.
## Indicators of Compromise
- File Hashes: N/A (The analysis relies on tool usage, not specific malware binaries)
- File Names: `gqrx.conf` (gqrx configuration file used for tuning)
- Registry Keys: N/A
- Network Indicators: N/A (Communication is short-range RF, though the tag stores Philips URL: `https://www.usa.philips.com/c-m-pe/toothbrush-heads` - defanged)
- Behavioral Indicators:
- Observation of specific NFC command structures (e.g., `0x1B` command associated with password transmission).
- Data changes observed at NFC memory address `0x24` upon device use.
- Serial communication attempts on the NFC layer resulting in write lockouts.
## Associated Threat Actors
- Individual Security Researcher / Hobbyist (This research was performed in a non-malicious context to understand device limitations).
- **Note:** Subsequent updates mention **Aaron Christophel** who reverse-engineered the firmware to solve the password generation puzzle and created a password generator tool on GitHub.
## Detection Methods
- **Signature-based detection:** Not highly applicable here, as the attack relies on analyzing legitimate protocol traffic.
- **Behavioral detection:** Monitoring for unauthorized access attempts or configuration changes on NFC tags associated with IoT devices. Detecting the presence of specialized SDR software (HackRF, Gnu Radio) interacting with standard NFC readers.
- **YARA rules:** Not applicable for this specific non-binary attack vector.
## Mitigation Strategies
- **Stronger NFC Security Implementations:** Utilizing hardware or software-based encryption/mutual authentication (beyond basic NTAG password protection) for critical device state data.
- **Unique Password Generation:** For devices relying on derived passwords, ensuring the one-way transformation function is complex or derived from a secure seed, rather than easily reversible sequences.
- **Limiting Write Access:** Implementing read-only configuration locks on critical counters, or utilizing hardware OTP (One-Time Programmable) memory, where available and applicable, for usage tracking.
- **Three Strikes Lockout Policy:** While useful, hardening the policy to prevent physical tampering from causing permanent writes/locks without a secure manufacturing process.
## Related Tools/Techniques
- **NFC Tools (Application):** Used for initial interrogation and subsequent password-protected write attempts.
- **HackRF:** Software Defined Radio hardware used for capturing raw RF signals.
- **Gnu Radio:** Software framework utilized to process raw IQ data into a usable WAV file format.
- **NFC-laboratory:** A tool specifically mentioned for taking the WAV file input and manually decoding the bytes from the modulated RF signal.
- **UID-based password generation:** Mentioned as the recommended NXP security approach, which the researcher failed to reverse engineer immediately (though later solved by others).