CVE-2025-15017

Executive Summary

CVE-2025-15017 exposes a critical hardware-level vulnerability in Moxa serial device servers. An active debug code remains enabled in the UART interface, allowing an attacker with physical access to connect directly, bypass authentication, and execute privileged operations. This results in a high impact to confidentiality, integrity, and availability of the affected device. No external system impact has been identified. The vulnerability is currently unpatched, and no public proof-of-concept or exploit code is available.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD base score not assigned; calculated vector below)
• Vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Physical (AV:P) – requires physical access to the device.
• Attack Complexity: Low (AC:L) – no special conditions needed.
• Privileges Required: None (PR:N).
• User Interaction: None (UI:N).
• Scope: Unchanged (S:U) – impact confined to the device.
• Confidentiality, Integrity, Availability: High (C:H/I:H/A:H).
• Threat Level: High (physical access required, but full device compromise).

Deep Dive & Mechanics

The vulnerability resides in the UART (Universal Asynchronous Receiver-Transmitter) interface of Moxa serial device servers. During development, debug code was left active in the firmware, exposing a privileged shell or debug console over the UART pins. An attacker with physical access can connect a serial adapter (e.g., USB-to-UART) to the device’s debug header, send a break signal or specific sequence, and gain a root-level shell without any authentication. This bypasses all security controls, allowing arbitrary command execution, firmware extraction, configuration modification, and denial-of-service attacks.

The root cause is a failure to disable debug interfaces in production firmware. The UART debug port remains active and unauthenticated, violating the principle of least privilege and secure development lifecycle practices.

Vulnerable vs. Patched Code

No patched version is available. The vulnerable code is the firmware’s UART initialization routine that leaves the debug console enabled. A patched version would disable the debug console or require authentication before granting shell access.

Vulnerable (pseudo-code):

void uart_init() {
    // Enable UART for debug
    uart_enable(UART_DEBUG_PORT);
    // No authentication check
    uart_set_handler(debug_shell_handler);
}

Patched (hypothetical):

void uart_init() {
    // Disable debug UART in production
    uart_disable(UART_DEBUG_PORT);
    // Or require authentication
    uart_set_handler(authenticated_shell_handler);
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Physical access to the Moxa device.
• USB-to-UART adapter (e.g., FTDI, CP2102) and serial terminal software (e.g., PuTTY, screen, minicom).
• Knowledge of UART pinout (typically VCC, GND, TX, RX) – often labeled on PCB or found via probing.

Steps:

1. Identify UART pins on the device PCB (look for 4-pin header or test points).
2. Connect USB-to-UART adapter: GND to GND, TX to RX, RX to TX (cross-over).
3. Open serial terminal at common baud rates (115200, 57600, 38400, 19200) with 8N1 settings.
4. Power on the device and observe boot messages. If debug shell is active, you will see a login prompt or shell.
5. Send a break signal (Ctrl+Break or serial break) to trigger debug mode.
6. Execute privileged commands (e.g., cat /etc/shadow, reboot, dd if=/dev/mtd0 of=/tmp/firmware.bin).

Example terminal session:

screen /dev/ttyUSB0 115200
# Wait for boot, then press Ctrl+Break
# Debug shell appears
# cat /etc/passwd

Blue Team

Detection:

• Monitor physical security logs for unauthorized access to device enclosures.
• Use tamper-evident seals on device chassis.
• Implement network-based monitoring for anomalous device behavior (e.g., unexpected reboots, configuration changes).
• Check for serial console activity via device logs (if available).

Prevention:

• Restrict physical access to devices (locked racks, secure rooms).
• Disable unused UART ports via firmware configuration if possible.
• Apply vendor patch when available.
• Use hardware security modules to detect physical tampering.

Indicators of Compromise (IOCs)

• Physical: Tampered device casing, broken seals, exposed UART pins.
• Network: Unexpected outbound connections from the device (e.g., to attacker C2).
• System: Modified configuration files, new user accounts, unexpected processes.
• Logs: Serial console login events (if logging enabled), device reboots outside maintenance windows.
• Firmware: Hash mismatch compared to known good firmware.

Detection Engineering

YARA Rule

rule CVE_2025_15017_UART_DEBUG {
    meta:
        description = "Detects firmware with active UART debug console"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-15017"
    strings:
        $debug_string = "debug_shell" ascii wide nocase
        $uart_init = "uart_enable" ascii wide nocase
        $break_handler = "break_handler" ascii wide nocase
    condition:
        any of them
}

Sigma Rule

title: Moxa Serial Device Server UART Debug Access
id: 8a7b3c2d-1e5f-4a6b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects physical access to UART debug interface on Moxa devices
author: Valters IT
date: 2025-04-10
logsource:
    product: moxa
    service: serial_device_server
detection:
    selection:
        EventID: 1001  # Example: device reboot or config change
        Message: "UART debug console accessed"
    condition: selection
falsepositives:
    - Authorized maintenance
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-15017 Moxa UART Debug Exploit Attempt"; flow:to_server; content:"|55 41 52 54|"; distance:0; within:4; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Physical Security: Restrict physical access to affected devices. Use tamper-evident seals and monitor access logs.
2. Firmware Hardening: If possible, disable UART debug interface via device configuration or firmware update.
3. Network Segmentation: Isolate serial device servers on a separate VLAN with strict firewall rules.
4. Monitoring: Enable logging on devices and monitor for unauthorized access attempts.

Long-Term:

• Apply vendor patch when released.
• Replace devices with models that have secure boot and disabled debug interfaces.
• Implement hardware security modules (HSM) for critical devices.

Vendor Contact: Moxa has been notified; status is UNKNOWN/UNPATCHED. Monitor Moxa security advisories for updates.

Valters IT