Run the NGINX Web Server as a non-root user

>Draft CIS Benchmark 1.1.1

## Description
Although NGINX master process is typically started with `root `privileges in order to listen on port 80 and 443, it can and should run as another non-root user in order to perform the web services. The NGINX `user `directive are used to designate the user that the NGINX worker processes will assume.

## Rationale
One of the best ways to reduce your exposure to attack when running a web server is to create a unique, unprivileged user and group for the server application. The "nobody" or "daemon" user and group that comes default on Unix variants should NOT be used to run the web server, since the account is commonly used for other separate daemon services. Instead, an account used only by the nginx software so as to not give unnecessary access to other services. Also the user identifier used for the nginx user should be a unique system account. System user accounts UID numbers have lower values which are reserved for the special system accounts not used by regular users. Typically system accounts numbers ranges from 1-999, or 1-499 and are defined in the `/etc/login.defs` file. As an even more secure alternative, if the NGINX web server can be run on high unprivileged ports, then it is not necessary to start NGINX as `root`, and all of the NGINX processes may be run as the NGINX specific user as described below.

## Remediation
Perform the following: 1. If the `nginx `user and group do not already exist, create the account and group as a unique system account: ``` # groupadd -r nginx # useradd nginx -r -g nginx -d /var/cache/nginx -s /sbin/nologin ``` 2. Configure the NGINX user in the NGINX configuration file `nginx.conf`: ``` user nginx; ```

## Audit
Ensure the nginx account is unique and has been created with a UID less than the minimum normal user account with the nginx group and configured in the nginx.conf file. 1. Ensure the previous line is present in the NGINX configuration and not commented out: ``` # grep -i '^user' $NGINX_PREFIX/nginx.conf ``` 2. Ensure the nginx account UID is correct: ``` # grep '^UID_MIN' /etc/login.defs # id nginx ```The uid must be less than the `UID\_MIN` value in `/etc/login.defs`, and group of `nginx `similar to the following entries: ``` UID_MIN 1000 uid=483(nginx) gid=479(nginx) groups=479(nginx) ``` 3. While the web server is running, check the user id for the `nginx `processes. The user name should match the configuration file. ``` # ps axu | grep nginx | grep -v '^root' ```