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What does it do with pc?

A backdoor is a hidden entrance into an application, network, or device. 
It’s a shortcut in a system that allows an authorized or unauthorized user to bypass security checks (like username and password authentication) to log in.
Hackers can install a backdoor onto your device by using malware such as a backdoor virus. 
Once inside, the damage possibilities are endless. 
The attackers can steal crucial data, spy on your activities, and target your clients.