Backdoor

A backdoor is a typically covert method of bypassing normal authentication or encryption in a computer, product, embedded device. 
It may be used to gain access to privileged information like passwords, corrupt or delete data on hard drives, or transfer information within networks. Backdoor is most often used for securing remote access to a computer.
Trojan horses can be used to create vulnerabilities in a system. A Trojan horse may appear to be an entirely legitimate program, but when executed, it triggers an activity that may install a backdoor. Although, some are secretly installed. 
Default passwords - It can be used to access the system as a backdoor, because if they are not changed by the user. Debugs - As backdoor if they are not removed in the release version.