> Backdoor users on Linux with uid=0

On Unix/Linux users with a uid=0 are root. This means any security checks are bypassed for them. An adversary might go ahead and create a new account, or set an existing account’s user identifier (uid) or group identifier to zero. A simple way to do this is to update /etc/passwd of an account, or use usermod -u 0 -o mallory. Let’s create a new user named mallory: wuzzi@saturn:/$ sudo adduser mallory [.