If you encounter the error WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! while connecting to your remote machine through SSH, it means the identification key on the server is not the same as the last connection. To fix this, simply remove the host's key from ~/.ssh/known_hosts.
The default file permission on Linux is 777, which means everyone can read, write and execute the file. It's fine for most of the scenario, but it's another story when it comes to SSH private key. If you try to establish an SSH connection with a too open private key, WARNING: UNPROTECTED PRIVATE KEY FILE! will be shown.