CVE-2023-4818

Executive Summary

CVE-2023-4818 is a vulnerability in the PAX A920 payment terminal that allows an attacker with physical USB access to downgrade the device’s bootloader. The vulnerability stems from a flawed version check in the bootloader update mechanism. Although the bootloader signature is correctly verified (only PAX-signed bootloaders are accepted), the version comparison logic can be bypassed, enabling the installation of an older, potentially exploitable bootloader version. This downgrade attack can undermine the device’s secure boot chain and expose the terminal to further compromise.

Affected Ecosystem: Embedded/IoT (PAX A920 payment terminal)
Attack Vector: Physical USB access required
Impact: Bootloader downgrade, potential bypass of secure boot, device integrity compromise

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a hardware/firmware-level issue in a specific embedded device.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official CVSS vector assigned)
• Attack Vector (AV): Physical (P) – Requires physical USB connection
• Attack Complexity (AC): Low (L) – No special conditions beyond USB access
• Privileges Required (PR): None (N) – No prior access needed
• User Interaction (UI): None (N) – No user action required
• Scope (S): Changed (C) – Downgrade affects the bootloader, impacting system integrity
• Confidentiality (C): None (N) – No direct data exposure
• Integrity (I): High (H) – Bootloader downgrade can lead to integrity compromise
• Availability (A): Low (L) – Downgrade may cause instability or bricking

Threat Level: Medium (Physical access required, but impact on secure boot chain is significant)

Deep Dive & Mechanics

The PAX A920 bootloader update process includes a version check to prevent downgrading to older, less secure versions. However, the version comparison logic contains a bug that allows an attacker to bypass this check. The attacker can present a validly signed but older bootloader image, and the flawed version check will accept it.

Key Technical Details:

• The bootloader is signed with PAX’s private key, and the signature is verified before installation.
• The version check is intended to compare the new version against the current version and reject downgrades.
• The bug likely involves an integer overflow, incorrect string comparison, or missing boundary check in the version parsing logic.
• Once an older bootloader is installed, the attacker can exploit known vulnerabilities in that version to gain further control.

Attack Flow:

1. Attacker gains physical access to the PAX A920 terminal.
2. Attacker connects via USB and sends a specially crafted bootloader update request.
3. The device verifies the signature (passes) but the version check is bypassed due to the bug.
4. The older bootloader is installed, potentially disabling security features or enabling further exploitation.

Vulnerable vs. Patched Code

No official patch or patched code is available. The vulnerability is in the bootloader version check logic. A hypothetical fix would involve:

Vulnerable Code (Conceptual):

int check_version(char *new_ver, char *cur_ver) {
    // Bug: Uses strcmp without proper length validation
    if (strcmp(new_ver, cur_ver) < 0) {
        return REJECT; // Downgrade detected
    }
    return ACCEPT;
}

Patched Code (Conceptual):

int check_version(char *new_ver, char *cur_ver) {
    // Proper version comparison using semantic versioning
    int new_major, new_minor, new_patch;
    int cur_major, cur_minor, cur_patch;
    sscanf(new_ver, "%d.%d.%d", &new_major, &new_minor, &new_patch);
    sscanf(cur_ver, "%d.%d.%d", &cur_major, &cur_minor, &cur_patch);
    if (new_major < cur_major ||
        (new_major == cur_major && new_minor < cur_minor) ||
        (new_major == cur_major && new_minor == cur_minor && new_patch < cur_patch)) {
        return REJECT;
    }
    return ACCEPT;
}

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• PAX A920 device with vulnerable bootloader
• USB cable
• Custom tool to craft bootloader downgrade payload (e.g., Python script using pyusb)

Exploitation Steps:

1. Identify the device via USB vendor ID (VID) and product ID (PID).
2. Extract the current bootloader version from the device.
3. Obtain a validly signed older bootloader image (e.g., from a firmware dump or leaked repository).
4. Craft a USB control transfer that sends the older bootloader image with a manipulated version string that bypasses the check.
5. Send the payload and trigger the bootloader update.
6. After downgrade, exploit known vulnerabilities in the older bootloader (e.g., buffer overflow, insecure memory access).

Example Python Script (Conceptual):

import usb.core
import usb.util

# Find PAX A920 device
dev = usb.core.find(idVendor=0x1234, idProduct=0x5678)
if dev is None:
    raise ValueError("Device not found")

# Craft bootloader downgrade payload
old_bootloader = open("old_bootloader.bin", "rb").read()
# Manipulate version string to bypass check (e.g., set to "99.99.99")
payload = b"VERSION=99.99.99\n" + old_bootloader

# Send via USB control transfer
dev.ctrl_transfer(bmRequestType=0x40, bRequest=0x01, wValue=0x00, wIndex=0x00, data_or_wLength=payload)

Blue Team (Defensive)

Detection & Prevention:

• Monitor USB activity on PAX A920 devices (e.g., via syslog or device logs).
• Implement physical security controls to prevent unauthorized USB access.
• Use tamper-evident seals on USB ports.
• Regularly audit bootloader versions on all devices.
• Deploy endpoint detection agents that can detect bootloader downgrade attempts.

Honeypot Strategy:

• Set up a honeypot PAX A920 device to capture attack patterns.
• Log all USB connection attempts and bootloader update requests.

Indicators of Compromise (IOCs)

• USB Vendor ID: 0x1234 (example, actual VID may vary)
• USB Product ID: 0x5678 (example, actual PID may vary)
• Bootloader Version: Any version lower than the latest known secure version (e.g., < 2.0.0)
• Network Traffic: No network IOCs (attack is physical)
• File Hashes: MD5/SHA256 of known older bootloader images (e.g., old_bootloader.bin)

Detection Engineering

YARA Rule

rule PAX_A920_Bootloader_Downgrade {
    meta:
        description = "Detects PAX A920 bootloader downgrade payload"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-4818"
    strings:
        $magic = "PAX_BOOTLOADER" ascii wide
        $version_bypass = "VERSION=99.99.99" ascii wide
    condition:
        $magic and $version_bypass
}

Sigma Rule

title: PAX A920 Bootloader Downgrade Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects USB control transfers targeting PAX A920 bootloader downgrade
author: Valters IT
logsource:
    category: usb
    product: windows
detection:
    selection:
        EventID: 2100  # USB control transfer event
        DeviceDescription: "PAX A920"
        TransferType: "Control"
        bmRequestType: 0x40
        bRequest: 0x01
    condition: selection
falsepositives:
    - Legitimate bootloader updates
level: high

Suricata Rule

alert usb any any -> any any (msg:"PAX A920 Bootloader Downgrade Attempt"; usb.bmRequestType:0x40; usb.bRequest:0x01; usb.wValue:0x00; usb.wIndex:0x00; content:"PAX_BOOTLOADER"; content:"VERSION=99.99.99"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Physical Security: Restrict physical access to PAX A920 devices. Use locked enclosures or tamper-evident seals.
2. Firmware Update: Contact PAX for an official patch. If unavailable, consider replacing devices with newer models.
3. Monitoring: Enable logging of USB activity and bootloader update attempts. Review logs regularly.

Long-Term Fixes:

• PAX must release a firmware update that fixes the version check logic.
• Implement cryptographic version binding (e.g., include version in the signed payload).
• Use hardware security modules (HSMs) to enforce bootloader version policies.

Workarounds:

• Disable USB bootloader update capability if not required.
• Use USB port blockers or disable USB ports via device configuration.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert