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Test and config files

kube-bench runs checks specified in controls files that are a YAML representation of the CIS Kubernetes Benchmark checks (or other distribution-specific hardening guides).

Controls

controls is a YAML document that contains checks that must be run against a specific Kubernetes node type, master or node and version.

controls is the fundamental input to kube-bench. The following is an example of a basic controls:

---
controls:
id: 1
text: "Master Node Security Configuration"
type: "master"
groups:
- id: 1.1
  text: API Server
  checks:
    - id: 1.1.1
      text: "Ensure that the --allow-privileged argument is set (Scored)"
      audit: "ps -ef | grep kube-apiserver | grep -v grep"
      tests:
      bin_op: or
      test_items:
      - flag: "--allow-privileged"
        set: true
      - flag: "--some-other-flag"
        set: false
      remediation: "Edit the /etc/kubernetes/config file on the master node and
        set the KUBE_ALLOW_PRIV parameter to '--allow-privileged=false'"
      scored: true
- id: 1.2
  text: Scheduler
  checks:
    - id: 1.2.1
      text: "Ensure that the --profiling argument is set to false (Scored)"
      audit: "ps -ef | grep kube-scheduler | grep -v grep"
      tests:
        bin_op: and
        test_items:
          - flag: "--profiling"
            set: true
          - flag: "--some-other-flag"
            set: false
      remediation: "Edit the /etc/kubernetes/config file on the master node and
        set the KUBE_ALLOW_PRIV parameter to '--allow-privileged=false'"
      scored: true

controls is composed of a hierarchy of groups, sub-groups and checks. Each of the controls components have an id and a text description which are displayed in the kube-bench output.

type specifies what kubernetes node type a controls is for. Possible values for type are master and node.

Groups

groups is a list of subgroups that test the various Kubernetes components that run on the node type specified in the controls.

For example, one subgroup checks parameters passed to the API server binary, while another subgroup checks parameters passed to the controller-manager binary.

groups:
- id: 1.1
  text: API Server
  # ...
- id: 1.2
  text: Scheduler
  # ...

These subgroups have id, text fields which serve the same purposes described in the previous paragraphs. The most important part of the subgroup is the checks field which is the collection of actual checks that form the subgroup.

This is an example of a subgroup and checks in the subgroup.

id: 1.1
text: API Server
checks:
  - id: 1.1.1
    text: "Ensure that the --allow-privileged argument is set (Scored)"
    audit: "ps -ef | grep kube-apiserver | grep -v grep"
    tests:
    # ...
  - id: 1.1.2
    text: "Ensure that the --anonymous-auth argument is set to false (Not Scored)"
    audit: "ps -ef | grep kube-apiserver | grep -v grep"
    tests:
    # ...

kube-bench supports running a subgroup by specifying the subgroup id on the command line, with the flag --group or -g.

Check

The CIS Kubernetes Benchmark recommends configurations to harden Kubernetes components. These recommendations are usually configuration options and can be specified by flags to Kubernetes binaries, or in configuration files.

The Benchmark also provides commands to audit a Kubernetes installation, identify places where the cluster security can be improved, and steps to remediate these identified problems.

In kube-bench, check objects embody these recommendations. This an example check object:

id: 1.1.1
text: "Ensure that the --anonymous-auth argument is set to false (Not Scored)"
audit: "ps -ef | grep kube-apiserver | grep -v grep"
tests:
  test_items:
  - flag: "--anonymous-auth"
    compare:
      op: eq
      value: false
    set: true
remediation: |
  Edit the API server pod specification file kube-apiserver
  on the master node and set the below parameter.
  --anonymous-auth=false
scored: false

A check object has an id, a text, an audit, a tests, remediation and scored fields.

kube-bench supports running individual checks by specifying the check's id as a comma-delimited list on the command line with the --check flag.

The audit field specifies the command to run for a check. The output of this command is then evaluated for conformance with the CIS Kubernetes Benchmark recommendation.

The audit is evaluated against criteria specified by the tests object. tests contain bin_op and test_items.

test_items specify the criteria(s) the audit command's output should meet to pass a check. This criteria is made up of keywords extracted from the output of the audit command and operations that compare these keywords against values expected by the CIS Kubernetes Benchmark.

There are three ways to run and extract keywords from the output of the command used, | Command | Output var | |---|---| | audit | flag | | audit_config | path | | audit_env | env |

flag is used when the keyword is a command-line flag. The associated audit command could be any binaries available on the system like ps command and a grep for the binary whose flag we are checking:

ps -ef | grep somebinary | grep -v grep

Here is an example usage of the flag option:

# ...
audit: "ps -ef | grep kube-apiserver | grep -v grep"
tests:
  test_items:
  - flag: "--anonymous-auth"
  # ...

path is used when the keyword is an option set in a JSON or YAML config file. The associated audit_command command is usually cat /path/to/config-yaml-or-json. For example:

# ...
text: "Ensure that the --anonymous-auth argument is set to false (Not Scored)"
audit: "cat /path/to/some/config"
tests:
  test_items:
  - path: "{.someoption.value}"
    # ...

env is used to check if the value is present within a specified environment variable. The presence of env is treated as an OR operation, if both flag and env are supplied it will use either to attempt pass the check. The command used for checking the environment variables of a process is generated by default.

If the command being generated is causing errors, you can override the command used by setting audit_env on the check. Similarly, if you don't want the environment checking command to be generated or run at all, specify disableEnvTesting as true on the check.

The example below will check if the flag --auto-tls is equal to false OR ETCD_AUTO_TLS is equal to false

  test_items:
  - flag: "--auto-tls"
    env: "ETCD_AUTO_TLS"
    compare:
      op: eq
      value: false
Note: flag, path and env will act as OR if more then one present.

test_item compares the output of the audit command and keywords using the set and compare fields.

  test_items:
  - flag: "--anonymous-auth"
    compare:
      op: eq
      value: false
    set: true

set checks if a keyword is present in the output of the audit command or a config file. The possible values for set are true and false.

If set is true, the check passes only if the keyword is present in the output of the audit command, or config file. If set is false, the check passes only if the keyword is not present in the output of the audit command, or config file. set is true by default.

compare has two fields op and value to compare keywords with expected value. op specifies which operation is used for the comparison, and value specifies the value to compare against.

To use compare, set must true. The comparison will be ignored if set is false

The op (operations) currently supported in kube-bench are: - eq: tests if the keyword is equal to the compared value. - noteq: tests if the keyword is unequal to the compared value. - gt: tests if the keyword is greater than the compared value. - gte: tests if the keyword is greater than or equal to the compared value. - lt: tests if the keyword is less than the compared value. - lte: tests if the keyword is less than or equal to the compared value. - has: tests if the keyword contains the compared value. - nothave: tests if the keyword does not contain the compared value. - regex: tests if the flag value matches the compared value regular expression. When defining regular expressions in YAML it is generally easier to wrap them in single quotes, for example '^[abc]$', to avoid issues with string escaping. - bitmask : tests if keyward is bitmasked with the compared value, common usege is for comparing file permissions in linux.

Omitting checks

If you decide that a recommendation is not appropriate for your environment, you can choose to omit it by editing the test YAML file to give it the check type skip as in this example:

  checks:
  - id: 2.1.1
    text: "Ensure that the --allow-privileged argument is set to false (Scored)"
    type: "skip"
    scored: true

No tests will be run for this check and the output will be marked [INFO].

Configuration and Variables

Kubernetes component configuration and binary file locations and names vary based on cluster deployment methods and Kubernetes distribution used. For this reason, the locations of these binaries and config files are configurable by editing the cfg/config.yaml file and these binaries and files can be referenced in a controls file via variables.

The cfg/config.yaml file is a global configuration file. Configuration files can be created for specific Kubernetes versions (distributions). Values in the version-specific config overwrite similar values in cfg/config.yaml.

For example, the kube-apiserver in Red Hat OCP distribution is run as hypershift openshift-kube-apiserver instead of the default kube-apiserver. This difference can be specified by editing the master.apiserver.defaultbin entry cfg/rh-0.7/config.yaml.

Below is the structure of cfg/config.yaml:

nodetype
  |-- components
    |-- component1
  |-- component1
    |-- bins
    |-- defaultbin (optional)
    |-- confs
    |-- defaultconf (optional)
    |-- svcs
    |-- defaultsvc (optional)
    |-- kubeconfig
    |-- defaultkubeconfig (optional)

Every node type has a subsection that specifies the main configuration items.

  • components: A list of components for the node type. For example master will have an entry for apiserver, scheduler and controllermanager.

Each component has the following entries:

  • bins: A list of candidate binaries for a component. kube-bench checks this list and selects the first binary that is running on the node.

If none of the binaries in bins list is running, kube-bench checks if the binary specified by defaultbin is running and terminates if none of the binaries in both bins and defaultbin is running.

The selected binary for a component can be referenced in controls using a variable in the form $<component>bin. In the example below, we reference the selected API server binary with the variable $apiserverbin in an audit command.

id: 1.1.1
 text: "Ensure that the --anonymous-auth argument is set to false (Scored)"
 audit: "ps -ef | grep $apiserverbin | grep -v grep"
 # ...
  • confs: A list of candidate configuration files for a component. kube-bench checks this list and selects the first config file that is found on the node. If none of the config files exists, kube-bench defaults conf to the value of defaultconf.

The selected config for a component can be referenced in controls using a variable in the form $<component>conf. In the example below, we reference the selected API server config file with the variable $apiserverconf in an audit command.

id: 1.4.1
  text: "Ensure that the API server pod specification file permissions are
  set to 644 or more restrictive (Scored)"
  audit: "/bin/sh -c 'if test -e $apiserverconf; then stat -c %a $apiserverconf; fi'"
  • svcs: A list of candidate unitfiles for a component. kube-bench checks this list and selects the first unitfile that is found on the node. If none of the unitfiles exists, kube-bench defaults unitfile to the value of defaultsvc.

The selected unitfile for a component can be referenced in controls via a variable in the form $<component>svc. In the example below, the selected kubelet unitfile is referenced with $kubeletsvc in the remediation of the check.

id: 2.1.1
  # ...
  remediation: |
    Edit the kubelet service file $kubeletsvc
    on each worker node and set the below parameter in KUBELET_SYSTEM_PODS_ARGS variable.
    --allow-privileged=false
    Based on your system, restart the kubelet service. For example:
    systemctl daemon-reload
    systemctl restart kubelet.service
  # ...

  • kubeconfig: A list of candidate kubeconfig files for a component. kube-bench checks this list and selects the first file that is found on the node. If none of the files exists, kube-bench defaults kubeconfig to the value of defaultkubeconfig.

    The selected kubeconfig for a component can be referenced in controls with a variable in the form $<component>kubeconfig. In the example below, the selected kubelet kubeconfig is referenced with $kubeletkubeconfig in the audit command.

    id: 2.2.1
  text: "Ensure that the kubelet.conf file permissions are set to 644 or
  more restrictive (Scored)"
  audit: "/bin/sh -c 'if test -e $kubeletkubeconfig; then stat -c %a $kubeletkubeconfig; fi'"
  # ...