SAF InSpec Profile Developer

Writing InSpec Controls

May 22, 2022About 5 min

Writing InSpec Controls

Now that you understand which methods map to each requirement, you're ready to write InSpec controls.

Security & Configuration Requirements

Recall from Section 4 that we want to verify that our NGINX instance had been configured to meet the following requirements:

1. NGINX version 1.10.3 or later.
2. The following NGINX modules should be installed:
   * `http_ssl`
   * `stream_ssl`
   * `mail_ssl`
3. The NGINX configuration file - `/etc/nginx/nginx.conf`- should:
   * be owned by the `root` user and group.
   * not be readable, writeable, or executable by others.

Requirement (1) - NGINX Version

The first requirement is for the NGINX version to be 1.10.3 or later.

We can check this using the InSpec cmp matcher.

Replace the contents of my_nginx/controls/example.rb with this:

control 'nginx-version' do
  impact 1.0
  title 'NGINX version'
  desc 'The required version of NGINX should be installed.'
  describe nginx do
    its('version') { should cmp >= '1.10.3' }
  end
end

The `nginx_conf` resource docs

nginx_confopen in new window

The test has an impact of 1.0, meaning it is most critical. A failure might indicate to the team that this issue should be resolved as soon as possible, likely by upgrading NGINX to a newer version. The test compares nginx.version against version 1.10.3.

cmp is one of InSpec's built-in matchersopen in new window. cmp understands version numbers and can use the operators ==, <, <=, >=, and >. cmp compares versions by each segment, not as a string. For example, "7.4" is less than than "7.30".

Next, run inspec exec to execute the profile on the remote target.

inspec exec /root/my_nginx -t docker://DOCKER_CONTAINER_ID

This will ouput:

  Profile: InSpec Profile (my_nginx)
  Version: 0.1.0
  Target:  docker://DOCKER_CONTAINER_ID

    ✔  nginx-version: NGINX version
       ✔  Nginx Environment version should cmp >= "1.10.3"


  Profile Summary: 1 successful control, 0 control failures, 0 controls skipped
  Test Summary: 1 successful, 0 failures, 0 skipped

You see that the test passes.

Requirement (2) - NGINX Modules are Installed

The second requirement verifies that our required modules are installed.

  • http_ssl
  • stream_ssl
  • mail_ssl

Append your control file to add this describe block:

control 'nginx-modules' do
  impact 1.0
  title 'NGINX modules'
  desc 'The required NGINX modules should be installed.'
  describe nginx do
    its('modules') { should include 'http_ssl' }
    its('modules') { should include 'stream_ssl' }
    its('modules') { should include 'mail_ssl' }
  end
end

The second control resembles the first; however, this version uses multiple its statements and the nginx.modules method. The nginx.modules method returns a list; the built-in include matcher verifies whether a value belongs to a given list.

Run inspec exec on the target.

$ inspec exec /root/my_nginx -t docker://DOCKER_CONTAINER_ID

Which again, will output:


  Profile: InSpec Profile (my_nginx)
  Version: 0.1.0
  Target:  docker://DOCKER_CONTAINER_ID

    ✔  nginx-version: NGINX version
       ✔  Nginx Environment version should cmp >= "1.10.3"
    ✔  nginx-modules: NGINX version
       ✔  Nginx Environment modules should include "http_ssl"
       ✔  Nginx Environment modules should include "stream_ssl"
       ✔  Nginx Environment modules should include "mail_ssl"


  Profile Summary: 2 successful controls, 0 control failures, 0 controls skipped
  Test Summary: 4 successful, 0 failures, 0 skipped

This time, both controls pass.

Requirement (3) - Permission on the nginx_conf file

The third requirement verifies that the NGINX configuration file, /etc/nginx/nginx.conf:

  • is owned by the root user and group.
  • is not be readable, writeable, or executable by others.

Append your control file to add this describe block:

control 'nginx-conf' do
  impact 1.0
  title 'NGINX configuration'
  desc 'The NGINX config file should owned by root, be writable only by owner, and not writeable or and readable by others.'
  describe file('/etc/nginx/nginx.conf') do
    it { should be_owned_by 'root' }
    it { should be_grouped_into 'root' }
    it { should_not be_readable.by('others') }
    it { should_not be_writable.by('others') }
    it { should_not be_executable.by('others') }
  end
end

The `file` resource docs

fileopen in new window

The third control uses the file resource. The first 2 tests use should to verify the root owner and group. The last 3 tests use should_not to verify that the file is not readable, writable, or executable by others.

Run inspec exec on the target.

$ inspec exec /root/my_nginx -t docker://DOCKER_CONTAINER_ID

Again, outputting:

  Profile: InSpec Profile (my_nginx)
  Version: 0.1.0
  Target:  docker://DOCKER_CONTAINER_ID

    ...
    ×  nginx-conf: NGINX configuration (1 failed)
       ✔  File /etc/nginx/nginx.conf should be owned by "root"
       ✔  File /etc/nginx/nginx.conf should be grouped into "root"
       ×  File /etc/nginx/nginx.conf should not be readable by others
       expected File /etc/nginx/nginx.conf not to be readable by others
       ✔  File /etc/nginx/nginx.conf should not be writable by others
       ✔  File /etc/nginx/nginx.conf should not be executable by others


  Profile Summary: 2 successful controls, 1 control failure, 0 controls skipped
  Test Summary: 8 successful, 1 failure, 0 skipped

This time you see a failure. You discover that /etc/nginx/nginx.conf is potentially readable by others. Because this control also has an impact of 1.0, your team may need to investigate further.

Remember, the first step, detect, is where you identify where the problems are so that you can accurately assess risk and prioritize remediation actions.

For the second step, correct, you can use a configuration management tool or some other automation framework to correct compliance failures for you.

You won't correct this issue in this module, but later you can check out the Integrated Compliance with Chefopen in new window track to learn more about how to correct compliance issues using Chef.

Refactoring the code to use Inputs

Your my_nginx profile is off to a great start. As your requirements evolve, you can add additional controls. You can also run this profile as often as you need to verify whether your systems remain in compliance.

Let's review the control file, example.rb.

control 'nginx-version' do
  impact 1.0
  title 'NGINX version'
  desc 'The required version of NGINX should be installed.'
  describe nginx do
    its('version') { should cmp >= '1.10.3' }
  end
end

control 'nginx-modules' do
  impact 1.0
  title 'NGINX modules'
  desc 'The required NGINX modules should be installed.'
  describe nginx do
    its('modules') { should include 'http_ssl' }
    its('modules') { should include 'stream_ssl' }
    its('modules') { should include 'mail_ssl' }
  end
end

control 'nginx-conf' do
  impact 1.0
  title 'NGINX configuration'
  desc 'The NGINX config file should owned by root, be writable only by owner, and not writeable or and readable by others.'
  describe file('/etc/nginx/nginx.conf') do
    it { should be_owned_by 'root' }
    it { should be_grouped_into 'root' }
    it { should_not be_readable.by('others') }
    it { should_not be_writable.by('others') }
    it { should_not be_executable.by('others') }
  end
end

Although these rules do what you expect, imagine your control file contains dozens or hundreds of tests. As the data you check for, such as the version or which modules are installed, evolve, it can become tedious to locate and update your tests. You may also find that you repeat the same data in across multiple control files.

One way to improve these tests is to use inputs. Inputs enable you to separate the logic of your tests from the data of your tests. Input files are typically expressed as a YAML file (files ending in .yaml or .yml).

Profile Inputs exist in your profile's main directory within the inspec.yml for global inputs to be used across all the controls in your profile.

Let's create the inspec.yml file for our profile:

name: my_nginx
title: InSpec Profile
maintainer: The Authors
copyright: The Authors
copyright_email: you@example.com
license: Apache-2.0
summary: An InSpec Compliance Profile
version: 0.1.0
supports:
  platform: os

inputs:
  - name: nginx_version
    type: string
    value: 1.10.3

To access an input you will use the input keyword. You can use this anywhere in your control code.

For example:

control 'nginx-version' do
  impact 1.0
  title 'NGINX version'
  desc 'The required version of NGINX should be installed.'
  describe nginx do
    its('version') { should cmp >= input('nginx_version') }
  end
end

For our next control we require specific modules

Example of adding an array object of servers:

inputs:
  - name: servers
    type: array
    value:
      - server1
      - server2
      - server3

Similarly as the above example we can edit our inspec.yml file like this:

name: my_nginx
title: InSpec Profile
maintainer: The Authors
copyright: The Authors
copyright_email: you@example.com
license: Apache-2.0
summary: An InSpec Compliance Profile
version: 0.1.0
supports:
  platform: os

inputs:
  - name: nginx_version
    type: string
    value: 1.10.3

  - name: nginx_modules
    type: array
    value:
      - http_ssl
      - stream_ssl
      - mail_ssl

Your control can be changed to look like this:

control 'nginx-modules' do
  impact 1.0
  title 'NGINX modules'
  desc 'The required NGINX modules should be installed.'
  required_modules = input('nginx_modules')
  describe nginx do
    required_modules.each do |required_module|
      its('modules') { should include required_module }
    end
  end
end

Multiple Inputs Example

To change your inputs for platform specific cases you can setup multiple input files.

For example, a inspec.yml:

inputs:
  - name: users
    type: array
    required: true

A YAML file named windows.yml

users:
  - Administrator
  - Guest
  - Randy

A YAML file named linux.yml

users:
  - root
  - shadow
  - rmadison

The control file:

control 'system-users' do
  impact 0.8
  desc 'Confirm the proper users are created on the system'

  describe users do
    its('usernames') { should eq input('users') }
  end
end

The following command runs the tests and applies the inputs specified, first, on the Linux system:

inspec exec examples/profile-input --input-file examples/windows.yml

And, on our Windows systems:

inspec exec examples/profile-input --input-file examples/linux.yml

Running the baseline straight from Github

In this module, we use NGINX for learning purposes. If you're interested in NGINX specifically, you may be interested in the MITRE nginx-srg-baselineopen in new window profile on GitHub.

To execute the GitHub profile on your target system, run this inspec exec command.

inspec exec https://github.com/mitre/nginx-baseline -t docker://DOCKER_CONTAINER_ID
Apache-2.0 | Copyright © 2022 - The MITRE Corporation
Copyright © 2022 Aaron Lippold