kube webhook certgen

Use within Kubernetes

Typically the Kube Webhook Certgen image is used within Helm charts as a pre-install or post-install hook. A different way to test this image is to simply create a Kubernetes Pod (and the associated Service Account) and run the kube-webhook-certgen binary to create a certificate and store it into a secret.

First create the k8s manifests. Make sure you replace registry.ghost-prod.alphabravo.io/ghost-base/kube-webhook-certgen:<tag> with the appropriate tag.

cat > test.yaml << EOF
---
apiVersion: v1
kind: ServiceAccount
metadata:
  name: kube-webhook-certgen-sa
  namespace: default
---
apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: kube-webhook-certgen-sa-role
  namespace: default
rules:
  - apiGroups: [""]
    resources: ["secrets"]
    verbs: ["get", "create", "update"]
---
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: kube-webhook-certgen-sa-rolebinding
  namespace: default
subjects:
  - kind: ServiceAccount
    name: kube-webhook-certgen-sa
    namespace: default
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: kube-webhook-certgen-sa-role
---
apiVersion: v1
kind: Pod
metadata:
  name: kube-webhook-certgen
  namespace: default
  labels:
    app: kube-webhook-certgen
spec:
  serviceAccountName: kube-webhook-certgen-sa
  restartPolicy: Never
  containers:
    - name: kube-webhook-certgen
      image: registry.ghost-prod.alphabravo.io/ghost-base/kube-webhook-certgen:<tag>
      imagePullPolicy: IfNotPresent
      args:
        - create
        - --host=localhost
        - --namespace=default
        - --secret-name=test-secret
      env:
        - name: GODEBUG
          value: "fips140=on,tlsmlkem=1"
EOF

After applying the manifest above you should see how Kubernetes creates the pod, executes the command and you should have a new certificate created into the test-secret Kubernetes secret.

$ kubectl apply -f test.yaml

serviceaccount/kube-webhook-certgen-sa created
role.rbac.authorization.k8s.io/kube-webhook-certgen-sa-role created
rolebinding.rbac.authorization.k8s.io/kube-webhook-certgen-sa-rolebinding created
pod/kube-webhook-certgen created

$ kubectl get all

NAME                       READY   STATUS      RESTARTS   AGE
pod/kube-webhook-certgen   0/1     Completed   0          7s

NAME                 TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)   AGE
service/kubernetes   ClusterIP   10.96.0.1    <none>        443/TCP   10d

$ kubectl get secret test-secret -o yaml | yq .data.ca | base64 -d
-----BEGIN CERTIFICATE-----
MIIBdTCCARygAwIBAgIRANEf/OaGkMTv9XowCCvjjBkwCgYIKoZIzj0EAwIwDzEN
MAsGA1UEChMEbmlsMTAgFw0yNjAyMDIxNTM3MDRaGA8yMTI2MDEwOTE1MzcwNFow
DzENMAsGA1UEChMEbmlsMTBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABFAkBUT4
jdc6W5d53HvBemZJusotEM9C1YJhg3Z7+/FR+4M7XsHnVK0bQ5uxxuOZc/0WbdHC
0oFgXrdBks78h1CjVzBVMA4GA1UdDwEB/wQEAwICBDATBgNVHSUEDDAKBggrBgEF
BQcDATAPBgNVHRMBAf8EBTADAQH/MB0GA1UdDgQWBBR6SAUCrjUMJyIdEbGc9nno
45WclTAKBggqhkjOPQQDAgNHADBEAiACJVDb9D3d2M28JlJ3r/TGgOqhcDvmiCOy
46O/jg/JtgIgdPkbNsIaj/L7FQ1G69SyPHGeOZpHUsehs/smjb1Jx2g=
-----END CERTIFICATE-----

Non-hardened images vs Ghost hardened images

Key differences

Why run as non-root?

Ghost hardened images prioritize security through defense in depth:

• Principle of least privilege: The tool doesn't need root privileges to function
• Container escape mitigation: Non-root users have fewer privileges if compromised
• Compliance ready: Meets security requirements that mandate non-root containers
• Pod Security Standards: Compatible with restricted pod security policies

Hardened image debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. Common debugging methods for applications built with Ghost hardened images include:

• Docker Debug to attach to containers
• Docker's Image Mount feature to mount debugging tools
• Ecosystem-specific debugging approaches

Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

For example, you can use Docker Debug:

docker debug registry.ghost-prod.alphabravo.io/ghost-base/kube-webhook-certgen:<tag>

or mount debugging tools with the Image Mount feature:

docker run --rm -it --pid container:my-container \
  --mount=type=image,source=registry.ghost-prod.alphabravo.io/ghost-base/busybox,destination=/dbg,ro \
  registry.ghost-prod.alphabravo.io/ghost-base/kube-webhook-certgen:<tag> /dbg/bin/sh

Image variants

Ghost hardened images come in different variants depending on their intended use.

• Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

  • Run as the nonroot user
  • Do not include a shell or a package manager
  • Contain only the minimal set of libraries needed to run the app

• Build-time variants typically include dev in the variant name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

  • Run as the root user
  • Include a shell and package manager
  • Are used to build or compile applications

• FIPS variants include fips in the variant name and tag. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations. The certificate generation in FIPS mode uses FIPS-validated cryptographic algorithms.

Migrate to a Ghost hardened image

To migrate your application to a Ghost hardened image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Ghost hardened image. This and a few other common changes are listed in the following table of migration notes.

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this is typically going to be an image tagged as dev because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as dev, your final runtime stage should use a non-dev image variant.

4. Install additional packages

   Ghost hardened images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshooting

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Ghost hardened images is to use Docker Debug to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. To avoid issues, configure your application to listen on port 1025 or higher inside the container, even if you map it to a lower port on the host. For example, docker run -p 80:8080 my-image will work because the port inside the container is 8080, and docker run -p 80:81 my-image won't work because the port inside the container is 81.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Ghost hardened images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Ghost hardened images and update your Dockerfile if necessary.