Executive Summary

The D-Link DIR-456U Hardware Revision A1 contains a hardcoded telnet backdoor. This means the device automatically starts a telnet service at boot with a fixed username "Alphanetworks" and a static password "whdrv01_dlob_dir456U". The telnet daemon uses a custom binary that accepts user credentials via a specific flag, and the login process validates these credentials using a simple string comparison. If an attacker on the local network uses these credentials, they can gain root shell access with full administrative control over the device.

Additionally, the device is End-of-Life (EOL), so it will not receive any security patches to fix this vulnerability.

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Compliance Impact

The vulnerability allows an unauthenticated attacker on the local network to gain root shell access with full administrative control due to a hardcoded telnet backdoor in the D-Link DIR-456U device. This level of unauthorized access can lead to compromise of sensitive data and systems.

Such unauthorized access and potential data breaches can negatively impact compliance with common standards and regulations like GDPR and HIPAA, which require protection of personal and sensitive information and mandate strict access controls.

Since the device is End-of-Life and will not receive patches, the risk remains unmitigated, further increasing the likelihood of non-compliance with these regulations.

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Impact Analysis

This vulnerability allows an unauthenticated attacker on the local network to gain root-level access to the device. With full administrative control, the attacker can manipulate device settings, intercept or redirect network traffic, install malicious software, or use the device as a foothold to attack other devices on the network.

Because the device is no longer supported and will not receive patches, the risk remains unmitigated, increasing the likelihood of exploitation.

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Detection Guidance

This vulnerability involves a hardcoded telnet backdoor on the D-Link DIR-456U Hardware Revision A1 device, which starts a telnet daemon at boot with a known username and password.

To detect this vulnerability on your network or system, you can scan for devices running a telnet service and attempt to connect using the hardcoded credentials.

• Use a network scanning tool like nmap to identify devices with open telnet ports (port 23): nmap -p 23 --open <target-ip-range>
• Attempt to connect to the telnet service using the hardcoded username and password: telnet <device-ip> and then login with username 'Alphanetworks' and password 'whdrv01_dlob_dir456U'.
• Check for the presence of the telnet daemon startup script /etc/init0.d/S80telnetd.sh and the configuration file /etc/config/image_sign on the device if you have local access.

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Mitigation Strategies

Since the device is End-of-Life (EOL) and will not receive patches, immediate mitigation steps focus on limiting exposure and access.

• Isolate the affected D-Link DIR-456U device from untrusted or public networks to prevent unauthorized local network access.
• Disable the telnet service if possible, for example by removing or renaming the /etc/init0.d/S80telnetd.sh script to prevent the telnet daemon from starting at boot.
• Replace the device with a supported model that receives security updates.
• Monitor network traffic for suspicious telnet connections and unauthorized access attempts.

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