Uart Console

uart-console

Use when physical access is available, and you need an interactive root shell without desoldering chips or risking firmware corruption.

1. Pin identification

• Visual: Look for 3-4 unpopulated through-holes or test pads near the SoC.
• Multimeter:
  1. GND: 0 V (continuity with shielding).
  2. VCC: 3.3 V or 5 V (constant).
  3. TX: Idle HIGH (3.3 V), fluctuates during boot.
  4. RX: High-impedance (floats to ~0 V or pull-up).
• JTAGulator: Use UART scan capability if pads are unlabeled/dense.

2. Connection

Rule: Do NOT connect VCC if the device is self-powered. This causes voltage collision.

• Connect: GND → GND, RX → TX_pad, TX → RX_pad.

# Try standard baud rates: 115200, 57600, 38400, 19200, 9600
screen /dev/ttyUSB0 115200
# or
minicom -D /dev/ttyUSB0 -b 115200

If output is garbled, cycle through baud rates. Check logic analyzer for precise rate if needed.

3. Bootloader exploitation (U-Boot)

Monitor output closely on power-on.

• Prompt: Hit any key to stop autoboot. Press immediately.

Key U-Boot commands:

printenv         # Dump environment variables (credentials, boot paths, keys)
md 0x80000000    # Hex dump memory (hunt for loaded keys/passwords)
boot             # Resume boot process

Drop to shell (root init bypass):

# Replace normal init with a shell to bypass authentication
setenv bootargs 'console=ttyS0,115200 root=/dev/mtdblock2 init=/bin/sh'
boot

4. Secure boot bypass patterns

If U-Boot enforces verified boot (CONFIG_SECUREBOOT):

1. Read signing keys from NAND/SPI mapping (sometimes unprotected).
2. Patch U-Boot env locally to drop CONFIG_SECUREBOOT (requires SPI write).
3. Voltage glitching on SoC VCC rail exactly during signature verification branching.