The issue is related to Incorrect Access Control in the D-Link DIR-2640-US router. When setting up PPPoE, the router starts the quagga process, which uses the default password and port, allowing an attacker to easily log in using telnet. This enables the attacker to modify routing information, monitor traffic of all devices under the router, hijack DNS, and conduct phishing attacks. The interface in question may be perceived as a backdoor due to its exposure.