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Physical access to a computer system presents a wealth of malicious opportunities for cyber attackers. Left alone with a device, attackers could exploit bootloader vulnerabilities or boot to a USB drive to install malicious payloads such as spyware or keyloggers, create rouge accounts with remote access, place trojanized application binaries for persistent remote access, and more. Physical attacks can covertly tap network infrastructure such as physical cables or ports to eavesdrop on sensitive data during transmission or gain direct access to hard drives to extract valuable information.
In this article, we will look at another type of attack that requires physical access to a device: the dark art of Universal Asynchronous Receiver/Transmitter (UART) hacking. We will examine the various ways that UART hacking can be leveraged to exploit an organization’s network at one of its most vulnerable points; from within network security devices (including security devices such as firewalls, IDS, and IPS) themselves. We will also describe what hardware vendors need to ensure their products are resilient to UART hacking.
What Is UART?
UART (Universal Asynchronous Receiver-Transmitter) is a communication interface found in most electronic devices such as smart TVs, printers, IoT smart home devices, network appliances such as routers, firewalls, and switches, gaming consoles, some smartphones, automobile electronics, medical devices, and industrial machinery. UART is a basic serial data transmission protocol for communication and interoperability between hardware components. In a nutshell, it's a physical port that allows shell access to a device’s embedded operating system (OS) installed directly on its hardware.
Almost every electronic integrated circuit (IC) board has a UART port on it for several practical reasons. During the development and testing phases of a device, UART ports allow engineers to directly monitor the system's internal processes. UART facilitates debugging and provides a way to execute shell commands on the embedded OS to import updated software, retrieve logs, error messages, and other essential information for diagnosing and resolving issues.
Without some form of serial connection, device developers would have no way to interact with firmware. Other comparable examples of serial protocols include Universal Serial Bus (USB), SPI (Serial Peripheral Interface), and CAN FD (CAN with flexible data rate) among others.
Here is what a UART port typically looks like on a modern device’s integrated circuit board. It’s pretty innocuous and would not stand out or be easily identified by an inexperienced observer.
The Practical Purpose Of UART Interfaces
Prototype Debugging and Testing: During the development and testing phases of electronic devices, having a UART port allows engineers to communicate with and monitor the system's internal processes. It facilitates debugging and provides a way to retrieve logs, error messages, and other essential information for diagnosing and resolving issues.
Firmware Updates: UART’s direct access to a command shell enables firmware updates to be uploaded into the device's memory. This is particularly useful for devices that might need periodic software updates or enhancements to fix bugs, add features, or address security vulnerabilities but do not have another form of user interface such as a web-admin panel.
Hardware interoperability: Components of integrated circuits such as hardware modules, chipsets, microcontrollers, and system-on-a-chip need a standard protocol for communicating with other hardware modules. UART serves as the bridge that allows these components to communicate using a common language.
How Hackers Can Abuse UART
UART hacking can provide unauthorized access to a device by exploiting the UART interface. UART hacking seeks to manipulate a target system through its UART interface to install malware, tap sensitive communication, or discover vulnerabilities that could impact devices that use the same firmware. UART ports provide direct access to a device’s embedded OS, and can therefore be leveraged by attackers with physical access to a device to run shell commands and investigate the embedded OS and applications on the device.
If the target device’s firmware does not implement strong security, an attacker can easily gain privileged access to extract firmware, modify how the device functions, steal sensitive information, or install malware within the firmware. Some devices store encryption keys in hardware to secure data. If an attacker gains UART access, they could potentially extract these keys and compromise the encrypted data.
UART hacking is also conducted by ethical security researchers to evaluate and responsibly disclose the security of a particular hardware product with the goal of improving its security. These security assessments can uncover vulnerabilities that impact entire wide groups of products. The findings allow device manufacturers the chance to close security gaps that can be exploited via UART.
Here are how attackers can abuse UART:
Modify system behavior: By sending specific commands through the UART interface, an attacker could alter the system's settings to install malware or mal-firmware to gain persistent remote access and unauthorized high-privileged access such as man-in-the-middle network position to extract sensitive data or modify communication contents.
Find existing vulnerabilities in a device's firmware: UART access can provide valuable insights into a device's firmware and underlying software, aiding in reverse engineering efforts to discover other vulnerabilities and security weaknesses in the product. If an attacker is able to discover vulnerabilities that can be exploited remotely they can use this information to craft targeted attacks against any instance of the targeted device or even similar products.
Tapping UART to access sensitive data: By intercepting the communication over the UART interface, an attacker might gain access to sensitive data being transmitted between components. This could include authentication credentials, encryption keys, or other confidential information exchanged within the device.
How Does UART Hacking Work?
UART hacking involves physically accessing the device’s circuit board, identifying the UART interface, and connecting to the UART pins using specialized tools, such as a UART-to-USB converter or a logic analyzer.
The typical steps involved in a UART hacking are:
Gain physical access to the target device
Identify the UART port on the target device’s integrated circuit board
Distinguish the UART output (TX/send) and input (RX/read) ports
Connect a specialized UART interface device
Determine the UART interface’s baud rate
Interact with the UART command shell to execute commands and investigate the firmware or embedded OS running on a device’s chipset
Identify vulnerabilities that can be exploited such as vulnerable service or software library versions or permissions that allow binary replacement to install malware
Mitigating Against UART Hacking
There are essentially two main inflection points that need to be defended to mitigate the risks of UART hacking. The first and most obvious of these is securing physical access to devices and using other deterrent controls such as video surveillance to gain non-repudiation with respect to activities surrounding hardware.
The second most critical aspect of securing UART exposure is for device vendors to implement secure boot processes, strong access controls for embedded OS, disable debugging interfaces in production devices, and employ encryption and access control measures to prevent unauthorized access to critical systems and data.
Although organizations do not typically influence a hardware vendor’s development practices and embedded OS security, making strong physical access controls the most feasible way to prevent UART hacking. However, in high-security environments with low-risk tolerance, security assessments can be used to investigate whether the hardware products in use are vulnerable to UART exploitation and determine what an advanced hacker could do if given physical access to the device or look for bugs that could be exploited externally.
Universal Asynchronous Receiver/Transmitter (UART) hacking depends on a physical attack vector and can be used to covertly compromise a wide range of devices including critical network security devices. By exploiting UART communication interfaces, attackers gain unauthorized shell access to a device’s embedded OS, allowing for command execution, firmware extraction, malware installation to manipulate a device’s functionality, or discovery of security gaps that can be exploited remotely.
It's important for organizations with strict risk requirements to implement strong physical access controls to protect devices and conduct security audits of critical hardware devices to determine their degree of susceptibility to UART hacking.
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