An issue was discovered on Tenda AC15 devices. A remote, unauthenticated attacker can make a request to /goform/telnet, creating a telnetd service on the device. This service is password protected; however, several default accounts exist on the device that are root accounts, which can be used to log in.
https://www.fidusinfosec.com/tenda-ac15-unauthenticated-telnetd-start-cve-2018-5770/