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Analyzing UART

1. UART internals

  • Designed for two different components on a device to talk to each other
  • Microcontroller chip in devices typically have UART pins
  • UART is a way of serial communication
  • Common serial channels (TTL, RS232, USB, PCI, HDMI, SPI, I2C, CAN)
    • TTL interface has 3-5 pins
    • RS-232 interface has a 9-pin connector, etc.
  • These interfaces are also used for debugging (testing connections) the device

  • UART Data packet
    • starting bit (usually 0)
    • message (any 8-bits)
    • parity bit (not used)
    • stop bit (usually 1)
  • UART Data packet format (usual configuration)
    • 8N1 — which means 8 message bits, no parity bit, and 1 stop bit
  • UART Port type
    • hardware-based or software-based
    • when there is not enough UART ports on chipset
      • UART is emulated via GPIO (means software UART)
      • it is a rare situation

2. Pentest objectives

  • Access debug logs that usually contains a booting sequence
  • (might) allow bootloader access while the device is booting
  • (might) provide unauthenticated root shell (console access)
  • (might) provide loading firmware over the network
  • and more

  • Custom bootloaders are designed to prevent from entering single user mode
    • but that can also be bypassed.

3. Tools

  • Multimeter
  • Logic analyzer (for analyzing data packet)
    • Saleae Logic Analyzer (or)
    • Open Workbench Logic Sniffer
  • UART Converter/Adapter (any supported FTDI based hardware)
    • USB-TTL (or)
    • BusPriate (or)
    • Attify Badge
    • baudrate.py
  • Communication
    • minicom (or)
    • screen (or)
    • Attify Badge GUI

4. Identify UART pinouts

  • Look for the shape online to learn
  • Visual inspection of the internal device components
    • looking for 3/4 pins or pads close to each other
    • rarely scattered across the board
  • UART pins

    Pin Function
    Transmit (Tx) Transmits data from the device to the target
    Receive (Rx) Receives data from the target to the device
    Ground (GND) Ground
    Voltage (Vcc) Usually either 3.3V or 5V
  • Chipset will have instructions on the connectors, otherwise use a
  • Multimeter
    • continuity test (for GND)
      • make sure the device is turned off
      • set the pointer to continuity
      • place the black probe on a ground surface (any metal part on device)
      • place the red probe on each of the four pads one by one
      • the pin is giving the beep sound is the GND
    • voltage difference (for the Tx, Rx, Vcc)
      • set the pointer to V-20
      • place the black probe to GND
      • turn device on
      • place the red probe over other pins
      • constant high voltage is our Vcc pin

      • reboot the device
      • place the red probe over the other pins
      • huge fluctuation in the voltage value is Tx pin
        • during the initial 10–15 seconds
      • lowest voltage during entire process is Rx
  • Confirm the findings by hooking up a logic analyzer

5. Connecting UART

  • A converter device for emulating a serial connection
    • to communicate two devices over UART/JTAG/I2C,SPI,GPIO
    • USB-TTL or BusPriate or Attify Badge
  • A proper voltage converter
    • i.e. when using a 3.3v serial device to a 5v serial device
    • serial connection emulators have built-in
  • Adapter (Attify Badge) pin layout

    Pin Function
    D0 Tx Transmit
    D1 Rx Receive
  • Connect the EEPROM pins to the adapter (Attify Badge) by using jumper wires

    UART Adapter
    Tx Rx
    Rx Tx
    GND GND
    Vcc is NOT connected
  • Connect the adapter to the analysis PC via USB
    • learn the mount point under /dev
    • by default mounted on COM port /dev/USB0

6. Command shell

  • Baud rate specifies the number of bits transmitted per second
    • data being transferred between devices = data rate
  • Identify the baud rate of the device
    • common rates: 9600, 38400, 19200, 57600, and 115200
    • otherwise, find it by trial and error

    baudrate.py

    • change baud rates while establishing a serial connection
    • visually inspect which baud rate gives the readable output
    • after detecting correct baud rate press Ctrl + C
  • Interacting with console
    • screen < mount point > < baud rate >

    screen /dev/ttyUSB0 38400

    • reboot the device
    • access debug logs of the device booting up process
    • right after booting, “busybox” is up
    • unauthenticated root shell on the device

Division of Network and Systems Engineering | KTH