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Using Terrascan as a Kubernetes Admission Controller

Overview

Terrascan can be integrated with K8s admissions webhooks. Admission controllers help you control what resources are created on a kubernetes cluster. By using terrascan as an admission controller, resources violating security policies can be blocked from getting created in a kubernetes cluster. Please check our blog for more details and instructions!

Steps to configure terrascan as an admission controller: - SSL certificates: You can use valid SSL certificates or create self signed certificates and have your kubernetes cluster trust it. - Create terrascan config file - Run terrascan in server mode - Make sure terrascan is accessible via HTTPS from the kubernetes API server. - Configure a ValidatingWebhookConfiguration resource in kubernetes cluster pointing to the terrascan server

Installation Guide

Create an instance

Your Terrascan instance has the following requirements for being able to scan K8s configurations.

  1. Be accessible via HTTPS. Make sure your cloud firewall is configured to allow this.
  2. Have a valid SSL certificate for the served domain name. To do that, choose one of our suggested methods:
    • Use a subdomain of your choosing (e.g dev-terrascan-k8s.accurics.com) and create a valid certificate for this subdomain through your SSL certificate provider. Let's Encrypt is a free, simple to use certificate authority you can use.
    • Use a reverse-proxy to serve SSL requests; for example, use Cloudflare Flexible to get a certificate by a trusted-CA to your self-signed certificate.
    • Generate a self-signed certificate and have your K8s cluster trust it. To add a trusted CA to ca-pemstore, as demonstrated in paraspatidar's blog post.
  3. Use the Terrascan docker as demonstrated in this document, or run it from the sources.

Run Terrascan in Server Mode

Run Terrascan docker image in your server using the following command:

 sudo docker run -p 443:9443 -v <DATA_PATH>:/data -u root -e K8S_WEBHOOK_API_KEY=<API_KEY> accurics/terrascan server --cert-path /data/cert.pem --key-path /data/key.pem -c /data/config.toml

<API_KEY> is a key used for authentication between your K8s environment and the Terrascan server. Generate your preferred key and use it here.

<DATA_PATH> is a directory path in your server where both the certificate and the private key .pem files are stored. In addition, this directory is used to save the webhook logs. (An SQLite file)

You can specify a config file that specifies which policies to use in the scan and which violations should lead to rejection. Policies below the [severity] level will be ignored. Policies below the [k8s-admission-control] denied-severity will be logged and displayed by terrascan, but will not lead to a rejected admission response to the k8s API server.

A config file example: config.toml

  [severity]
  level = "medium"
  [rules]
      skip-rules = [
          "accurics.kubernetes.IAM.107"
      ]

  [k8s-admission-control]
    denied-categories = [
        "Network Ports Security"
    ]
    denied-severity = "high"
    dashboard=true

You can specify the following configurations:

  • scan-rules - one or more rules to scan
  • skip-rules - one or more rules to skip while scanning
  • severity - the minimal level of severity of the policies to be scanned and displayed. Options are high, medium and low
  • category - the list of type of categories of the policies to be scanned and displayed

k8s-admission-control - Config options for K8s Admission Controllers and GitOps workflows:

  • denied-severity - Violations of this or higher severity will cause and admission rejection. Lower severity violations will be warnings. Options are high, medium. and low
  • denied-categories - violations from these policy categories will lead to an admission rejection. Policy violations of other categories will lead to warnings.
  • dashboard=true - enable the /logs endpoint to log and graphically display admission requests and violations. Default is false

Configure a ValidatingWebhookConfiguration Resource in Kubernetes Cluster

Configure a new ValidatingWebhookConfiguration in your Kubernetes environment and specify your Terrascan server endpoint.

Example:

  cat <<EOF | kubectl apply -f -
  apiVersion: admissionregistration.k8s.io/v1
  kind: ValidatingWebhookConfiguration
  metadata:
    name: my.validation.example.check
  webhooks:
    - name: my.validation.example.check
      rules:
        - apiGroups:
            - ""
          apiVersions:
            - v1
          operations:
            - CREATE
            - UPDATE
          resources:
            - pods
            - services
      failurePolicy: Fail
      clientConfig:
        url: https://<SERVER_ADDRESS>/v1/k8s/webhooks/<API_KEY>/scan
      sideEffects: None
      admissionReviewVersions: ["v1"]
  EOF
  • You can modify the rules that trigger the webhook according to your preferences.
  • Update the clientConfig URL with your terrascan server address and the API key you generated before.

Test your settings

Try to run a new pod / service. For example:

  kubectl run mynginx --image=nginx

Go to https://<SERVER_ADDRESS>/k8s/webhooks/<API_KEY>/logs and verify your request is logged.