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Explanation:
Below is the CKS exam style "do-this-exactly" runbook for Q3. It includes the minimal discovery commands (so you don't guess filenames), then the exact lines/blocks to set.
QUESTION 3 - ImagePolicyWebhook (Validating Admission) - Exam Steps
0) SSH + root
ssh cks000002
sudo -i
1) Identify the provided config files (no guessing)
ls -la /etc/kubernetes/bouncer
You are looking for files typically named like:
admission_configuration.yaml (AdmissionConfiguration)
imagepolicywebhook.yaml (ImagePolicyWebhookConfiguration) OR the ImagePolicyWebhook config embedded inside the AdmissionConfiguration kubeconfig (webhook kubeconfig) If unsure which is which, quick peek:
grep -R "ImagePolicyWebhook" -n /etc/kubernetes/bouncer
grep -R "AdmissionConfiguration" -n /etc/kubernetes/bouncer
grep -R "kubeconfig" -n /etc/kubernetes/bouncer
PART A - Reconfigure API Server to enable required admission plugin(s)
2) Edit API server static pod manifest
vi /etc/kubernetes/manifests/kube-apiserver.yaml
2.1 Enable the admission plugin ImagePolicyWebhook
Find the line starting with:
- --enable-admission-plugins=
Ensure ImagePolicyWebhook is included in that comma list.
Example (your list may differ; just add ImagePolicyWebhook):
- --enable-admission-plugins=NodeRestriction,ImagePolicyWebhook
If the flag does not exist, add one line under command::
- --enable-admission-plugins=ImagePolicyWebhook
2.2 Point API server to the provided AdmissionConfiguration
In the same file, ensure this flag exists (use the file in /etc/kubernetes/bouncer that contains AdmissionConfiguration):
- --admission-control-config-file=/etc/kubernetes/bouncer/admission_configuration.yaml If your file is named differently, use the real filename you found in step 1, but keep the flag name exactly --admission-control-config-file.
Save/exit:
:wq
Static pod will restart automatically (kubelet watches the manifest).
Optional quick watch:
docker ps | grep kube-apiserver
# or:
crictl ps | grep kube-apiserver
PART B - Configure ImagePolicyWebhook to deny images on backend failure
3) Edit the ImagePolicyWebhook config
One of these is true on your cluster:
Option 1 (most common in these tasks): ImagePolicyWebhook config is a standalone file Edit the file in /etc/kubernetes/bouncer that contains kind: ImagePolicyWebhookConfiguration:
grep -R "kind: ImagePolicyWebhookConfiguration" -n /etc/kubernetes/bouncer vi /etc/kubernetes/bouncer/<THE_FILE_YOU_FOUND>.yaml Set (or ensure) exactly:
defaultAllow: false
Option 2: ImagePolicyWebhook config is embedded inside AdmissionConfiguration Edit the AdmissionConfiguration file:
vi /etc/kubernetes/bouncer/admission_configuration.yaml
Find the plugin section for ImagePolicyWebhook and ensure the config includes:
defaultAllow: false
✅ Save/exit:
:wq
PART C - Point backend configuration to https://smooth-yak.local/review
4) Edit the webhook kubeconfig to use the scanner endpoint
Find the kubeconfig file referenced by the ImagePolicyWebhook config.
Search for kubeConfigFile:
grep -R "kubeConfigFile" -n /etc/kubernetes/bouncer
Open that kubeconfig path (example name below; yours may differ):
vi /etc/kubernetes/bouncer/kubeconfig
In kubeconfig, set the cluster server exactly:
clusters:
- cluster:
server: https://smooth-yak.local/review
✅ Save/exit:
:wq
PART D - Restart effect (make sure API server picks up config)
Because you already edited /etc/kubernetes/manifests/kube-apiserver.yaml, the API server restarted.
To be safe (and fast), force a restart by "touching" the manifest (no content change needed):
touch /etc/kubernetes/manifests/kube-apiserver.yaml
PART E - Test: apply vulnerable workload and confirm it is denied
5) Use admin kubeconfig (because old kubectl config may break)
export KUBECONFIG=/etc/kubernetes/admin.conf
kubectl get nodes
6) Deploy the test resource (should be DENIED)
kubectl apply -f /home/candidate/vulnerable.yaml
Expected: admission error/denied message.
If it already exists:
kubectl delete -f /home/candidate/vulnerable.yaml
kubectl apply -f /home/candidate/vulnerable.yaml
PART F - Verify the scanner was called (log check)
7) Check scanner access log
tail -n 50 /var/log/nginx/access_log
You should see requests hitting /review.
Quick "what to check if it doesn't deny"
Run these in order:
Confirm API server flags:
grep -n "enable-admission-plugins" /etc/kubernetes/manifests/kube-apiserver.yaml grep -n "admission-control-config-file" /etc/kubernetes/manifests/kube-apiserver.yaml Confirm deny-on-failure:
grep -R "defaultAllow" -n /etc/kubernetes/bouncer
Must show:
defaultAllow: false
Confirm endpoint:
grep -R "server: https://smooth-yak.local/review" -n /etc/kubernetes/bouncer API server logs (docker runtime):
docker ps | grep kube-apiserver
docker logs $(docker ps -q --filter name=kube-apiserver) --tail 80
If you paste the output of:
ls -/etc/kubernetes/bouncer
grep -R "kind: AdmissionConfiguration" -n /etc/kubernetes/bouncer
grep -R "ImagePolicyWebhook" -n /etc/kubernetes/bouncer

A service account provides an identity for processes that run in a Pod.
When you (a human) access the cluster (for example, using kubectl), you are authenticated by the apiserver as a particular User Account (currently this is usually admin, unless your cluster administrator has customized your cluster). Processes in containers inside pods can also contact the apiserver. When they do, they are authenticated as a particular Service Account (for example, default).
When you create a pod, if you do not specify a service account, it is automatically assigned the default service account in the same namespace. If you get the raw json or yaml for a pod you have created (for example, kubectl get pods/<podname> -o yaml), you can see the spec.serviceAccountName field has been automatically set.
You can access the API from inside a pod using automatically mounted service account credentials, as described in Accessing the Cluster. The API permissions of the service account depend on the authorization plugin and policy in use.
In version 1.6+, you can opt out of automounting API credentials for a service account by setting automountServiceAccountToken: false on the service account:
apiVersion: v1
kind: ServiceAccount
metadata:
name: build-robot
automountServiceAccountToken: false
...
In version 1.6+, you can also opt out of automounting API credentials for a particular pod:
apiVersion: v1
kind: Pod
metadata:
name: my-pod
spec:
serviceAccountName: build-robot
automountServiceAccountToken: false
...
The pod spec takes precedence over the service account if both specify a automountServiceAccountToken value.

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[desk@cli] $ k create sa backend-qa -n qa
sa/backend-qa created
[desk@cli] $ k get role,rolebinding -n qa
No resources found in qa namespace.
[desk@cli] $ k create role backend -n qa --resource pods,namespaces,configmaps --verb list
# No access to secret
[desk@cli] $ k create rolebinding backend -n qa --role backend --serviceaccount qa:backend-qa
[desk@cli] $ vim /home/cert_masters/frontend-pod.yaml
apiVersion: v1
kind: Pod
metadata:
name: frontend
spec:
serviceAccountName: backend-qa # Add this
image: nginx
name: frontend
[desk@cli] $ k apply -f /home/cert_masters/frontend-pod.yaml
pod created
[desk@cli] $ k create sa backend-qa -n qa
serviceaccount/backend-qa created
[desk@cli] $ k get role,rolebinding -n qa
No resources found in qa namespace.
[desk@cli] $ k create role backend -n qa --resource pods,namespaces,configmaps --verb list role.rbac.authorization.k8s.io/backend created
[desk@cli] $ k create rolebinding backend -n qa --role backend --serviceaccount qa:backend-qa rolebinding.rbac.authorization.k8s.io/backend created
[desk@cli] $ vim /home/cert_masters/frontend-pod.yaml
apiVersion: v1
kind: Pod
metadata:
name: frontend
spec:
serviceAccountName: backend-qa # Add this
image: nginx
name: frontend
[desk@cli] $ k apply -f /home/cert_masters/frontend-pod.yaml
pod/frontend created
https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/