Kubernetes Admission Controller Webhook Demo

This repository contains a small HTTP server that can be used as a Kubernetes MutatingAdmissionWebhook.

The logic of this demo webhook is fairly simple: it enforces more secure defaults for running containers as non-root user. While it is still possible to run containers as root, the webhook ensures that this is only possible if the setting runAsNonRoot is explicitly set to false in the securityContext of the Pod. If no value is set for runAsNonRoot, a default of true is applied, and the user ID defaults to 1234.

Prerequisites

A cluster on which this example can be tested must be running Kubernetes 1.9.0 or above, with the admissionregistration.k8s.io/v1beta1 API enabled. You can verify that by observing that the following command produces a non-empty output:

kubectl api-versions | grep admissionregistration.k8s.io/v1beta1

In addition, the MutatingAdmissionWebhook admission controller should be added and listed in the admission-control flag of kube-apiserver.

For building the image, GNU make and Go are required.

Deploying the Webhook Server

1. Bring up a Kubernetes cluster satisfying the above prerequisites, and make sure it is active (i.e., either via the configuration in the default location, or by setting the KUBECONFIG environment variable).
2. Run ./deploy.sh. This will create a CA, a certificate and private key for the webhook server, and deploy the resources in the newly created webhook-demo namespace in your Kubernetes cluster.

Verify

1. The webhook-server pod in the webhook-demo namespace should be running:

$ kubectl -n webhook-demo get pods
NAME                             READY     STATUS    RESTARTS   AGE
webhook-server-6f976f7bf-hssc9   1/1       Running   0          35m

2. A MutatingWebhookConfiguration named demo-webhook should exist:

$ kubectl get mutatingwebhookconfigurations
NAME           AGE
demo-webhook   36m

3. Deploy a pod that neither sets runAsNonRoot nor runAsUser:

$ kubectl create -f examples/pod-with-defaults.yaml

Verify that the pod has default values in its security context filled in:

$ kubectl get pod/pod-with-defaults -o yaml
...
  securityContext:
    runAsNonRoot: true
    runAsUser: 1234
...

Also, check the logs that the pod had in fact been running as a non-root user:

$ kubectl logs pod-with-defaults
I am running as user 1234

4. Deploy a pod that explicitly sets runAsNonRoot to false, allowing it to run as the root user:

$ kubectl create -f examples/pod-with-override.yaml
$ kubectl get pod/pod-with-override -o yaml
...
  securityContext:
    runAsNonRoot: false
...
$ kubectl logs pod-with-override
I am running as user 0

5. Attempt to deploy a pod that has a conflicting setting: runAsNonRoot set to true, but runAsUser set to false. The admission controller should block the creation of that pod.

$ kubectl create -f examples/pod-with-conflict.yaml 
Error from server (InternalError): error when creating "examples/pod-with-conflict.yaml": Internal error
occurred: admission webhook "webhook-server.webhook-demo.svc" denied the request: runAsNonRoot specified,
but runAsUser set to 0 (the root user)

Build the Image from Sources (optional)

An image can be built by running make. If you want to modify the webhook server for testing purposes, be sure to set and export the shell environment variable IMAGE to an image tag for which you have push access. You can then build and push the image by running make push-image. Also make sure to change the image tag in deployment/deployment.yaml.template, and if necessary, add image pull secrets.