Depending on your Linux flavor, the ssh daemon might listen on all network interfaces on the default port, and allow root and password logins.
This default configuration is not very safe. Automated scripts can try to guess the root password. You're at the mercy of the strength of your root password.
It's a good idea to make things stricter. Let's see how you can do this.
Create a user who can log in using his ssh key instead of a password. Doing this with Chef is described in the Creating users from data bags recipe in this chapter.
Make sure that you have a cookbook named my_cookbook and that the run_list of your node includes my_cookbook, as described in the Creating and using cookbooks recipe in Chapter 1, Chef Infrastructure.
Create Berksfile in your Chef repository including my_cookbook:
mma@laptop:~/chef-repo $ subl Berksfile
cookbook 'my_cookbook', path: './cookbooks/my_cookbook'