The difference between an ethical hacker and a criminal is written permission . Any course on trojans/backdoors that doesn’t hammer this point is malpractice.
: A backdoor is a method of bypassing normal authentication or encryption to gain remote access to a system. It allows an attacker to return to a compromised machine at any time without having to re-exploit the initial vulnerability. Key Learning Objectives in Expert Video Series ethical hacking: trojans and backdoors [author] videos