CVE-2025-26409

Executive Summary

CVE-2025-26409 is a critical hardware-level vulnerability affecting Wattsense Bridge devices. The flaw allows an attacker with physical access to the device’s printed circuit board (PCB) to connect to a serial interface, gaining access to the bootloader and a Linux login prompt. From the bootloader, a root shell can be obtained, leading to full device compromise. This vulnerability is particularly severe in IoT/OT environments where physical security may be lax. The issue is patched in firmware BSP version 6.4.1 and later.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (Unscored by NVD) Attack Vector: Physical (AV:P) Attack Complexity: Low (AC:L) Privileges Required: None (PR:N) User Interaction: None (UI:N) Scope: Unchanged (S:U) Confidentiality Impact: High (C:H) Integrity Impact: High (I:H) Availability Impact: High (A:H)

Threat Level: Critical (based on impact, though CVSS is unscored). Physical access is required, but once obtained, full device takeover is trivial.

Deep Dive & Mechanics

The vulnerability lies in the hardware design of the Wattsense Bridge PCB. A serial interface (likely UART) is exposed on the board without proper authentication or obfuscation. When an attacker connects to this interface using a USB-to-serial adapter (e.g., FTDI cable), they are presented with two options:

1. Bootloader Access: Allows modification of boot parameters, kernel loading, and potentially flashing malicious firmware.
2. Linux Login Prompt: Provides a standard Linux shell, but with default or weak credentials (or no password required) leading to root access.

The bootloader access is particularly dangerous because it can be used to bypass any software-based security controls, install persistent backdoors, or extract sensitive data from the device’s storage.

Vulnerable vs. Patched Code

Vulnerable (BSP < 6.4.1):

• The serial interface is enabled and accessible without authentication.
• The bootloader (likely U-Boot) does not require a password for interactive access.
• The Linux login prompt may have default credentials or no password for the root user.

Patched (BSP >= 6.4.1):

• The serial interface is disabled or requires authentication before granting access.
• The bootloader is configured to require a password for interactive commands.
• The Linux login prompt enforces strong authentication for all users.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Physical access to a Wattsense Bridge device (BSP < 6.4.1).
• USB-to-serial adapter (e.g., FTDI TTL-232R-3V3).
• Terminal emulator (e.g., PuTTY, screen, minicom).
• Screwdriver to open the device enclosure.

Steps:

1. Open the Device: Remove screws or clips to access the PCB.
2. Identify Serial Pins: Locate the UART header (typically 4 pins: GND, TX, RX, VCC). Use a multimeter to confirm.
3. Connect Adapter: Connect the USB-to-serial adapter to the UART pins (GND to GND, TX to RX, RX to TX, VCC optional).
4. Establish Connection: Use a terminal emulator with baud rate 115200 (common for embedded devices).
5. Interrupt Boot: Press a key (e.g., Enter or Space) during boot to enter the bootloader prompt.
6. Gain Root Shell: From the bootloader, use commands like boot or run to load a custom kernel, or simply wait for the Linux login prompt and log in as root (no password).

Example Bootloader Commands:

# Interrupt boot and enter U-Boot prompt
# Then:
setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p2 rw
boot

Blue Team

Detection & Prevention:

• Physical Security: Enclose devices in tamper-evident cases and monitor for unauthorized access.
• Firmware Updates: Ensure all devices are updated to BSP >= 6.4.1.
• Serial Port Disable: If possible, disable the serial interface in the bootloader configuration.
• Bootloader Password: Set a strong password for the bootloader.
• Logging: Enable audit logging for any serial console access attempts.

Indicators of Compromise (IOCs)

• Physical: Tamper evidence on device enclosure (broken seals, scratches).
• Network: Unusual outbound connections from the device (e.g., to unknown IPs).
• System: Presence of unauthorized files in /root or /tmp.
• Bootloader: Modified boot parameters (check via printenv in U-Boot).
• Processes: Unknown processes running as root (e.g., reverse shells).

Detection Engineering

YARA Rule

rule Wattsense_Bridge_Serial_Exploit {
    meta:
        description = "Detects attempts to exploit CVE-2025-26409 via serial interface"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26409"
    strings:
        $bootloader = "U-Boot" nocase
        $linux_prompt = "login:" nocase
        $root_shell = "root@" nocase
    condition:
        any of them
}

Sigma Rule

title: Wattsense Bridge Serial Interface Access
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects serial console login attempts on Wattsense Bridge devices
author: Valters IT
date: 2025-04-10
logsource:
    product: linux
    service: auth
detection:
    selection:
        - EventID: 558 (Login via console)
        - Message: "root login on console"
    condition: selection
falsepositives:
    - Legitimate administrative access
level: high

Suricata Rule

alert tcp any any -> any 22 (msg:"Wattsense Bridge SSH Brute Force"; flow:to_server; content:"SSH-2.0"; nocase; threshold: type both, track by_src, count 5, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Update Firmware: Upgrade all Wattsense Bridge devices to BSP version 6.4.1 or later.
2. Physical Access Control: Restrict physical access to devices to authorized personnel only.
3. Tamper Detection: Implement tamper-evident seals and monitor for breaches.

Long-Term Measures:

• Hardware Security: In future designs, disable serial interfaces in production or require authentication.
• Bootloader Security: Enable signed boot images and password-protected bootloader access.
• Monitoring: Deploy SIEM rules to detect unauthorized console logins.

Valters IT
Securing IoT/OT Environments