- A
Enforce a PodSecurityStandard that restricts containers from running with root privileges.
Why wrong: Does not restrict image registry.
- B
Apply a Kubernetes NetworkPolicy that blocks egress traffic from nodes to Docker Hub.
Why wrong: NetworkPolicy applies to pods, not nodes; kubelet pulls images before the pod network is set up.
- C
Configure the containerd configuration on each node to use Harbor as a mirror for all registries and set endpoint to Harbor only, disabling direct pull from public registries.
This forces all image pulls to go through Harbor, and if the image is not cached or allowed, the pull fails.
- D
Deploy an admission webhook (e.g., OPA/Gatekeeper) that denies pods whose image registry is not the internal Harbor.
Why wrong: Admission controllers run at pod creation time, but if the image pull later fails due to network restrictions, the pod remains in ImagePullBackOff; however, the admission controller can be effective, but it requires policy and may not block all existing workloads. Option B is more robust because it stops the pull at the kubelet level, affecting even existing pods that might get rescheduled.
CKS Supply Chain Security Practice Question
This CKS practice question tests your understanding of supply chain security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
You are a security engineer at a fintech startup. The company runs a Kubernetes cluster in production with hundreds of microservices. Recently, a container image from a public registry was compromised, and the attacker injected a backdoor that exfiltrated customer data. The CISO mandates that all images must come from an internal registry that only stores approved, scanned, and signed images. Currently, developers build images locally and push them to Docker Hub, then reference those images in Kubernetes manifests. You have deployed Harbor as a private registry with vulnerability scanning and Cosign for signing. However, you notice that some pods are still running images directly from Docker Hub. You need to enforce that only images from your internal Harbor registry can be used in the cluster. You cannot change the Kubernetes manifests immediately because of a large backlog. You have access to the cluster's kubelet configuration and can modify cluster-level components. Which single action will most effectively block any pod that tries to use an image not hosted on your internal registry?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"immediately / without restart"Why it matters: Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Configure the containerd configuration on each node to use Harbor as a mirror for all registries and set endpoint to Harbor only, disabling direct pull from public registries.
Option C is correct because configuring containerd to use Harbor as a mirror for all registries and setting the endpoint to Harbor only effectively blocks pulls from any external registry at the container runtime level. This approach works even if Kubernetes manifests reference Docker Hub images, as the kubelet will redirect all image pull requests to the internal Harbor registry, preventing direct pulls from public registries. It enforces the policy without requiring immediate changes to existing manifests, which aligns with the constraint of not modifying them due to a large backlog.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enforce a PodSecurityStandard that restricts containers from running with root privileges.
Why it's wrong here
Does not restrict image registry.
- ✗
Apply a Kubernetes NetworkPolicy that blocks egress traffic from nodes to Docker Hub.
Why it's wrong here
NetworkPolicy applies to pods, not nodes; kubelet pulls images before the pod network is set up.
- ✓
Configure the containerd configuration on each node to use Harbor as a mirror for all registries and set endpoint to Harbor only, disabling direct pull from public registries.
Why this is correct
This forces all image pulls to go through Harbor, and if the image is not cached or allowed, the pull fails.
Clue confirmation
The clue word "immediately / without restart" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Deploy an admission webhook (e.g., OPA/Gatekeeper) that denies pods whose image registry is not the internal Harbor.
Why it's wrong here
Admission controllers run at pod creation time, but if the image pull later fails due to network restrictions, the pod remains in ImagePullBackOff; however, the admission controller can be effective, but it requires policy and may not block all existing workloads. Option B is more robust because it stops the pull at the kubelet level, affecting even existing pods that might get rescheduled.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CNCF often tests the distinction between admission control (webhooks) and runtime enforcement (container runtime configuration), where candidates mistakenly choose an admission webhook because it seems like a direct policy enforcement tool, but the question explicitly states that manifests cannot be changed immediately, making runtime-level enforcement the only viable option to block pulls without modifying existing resources.
Detailed technical explanation
How to think about this question
Configuring containerd's `[plugins."io.containerd.grpc.v1.cri".registry.mirrors]` section with a `endpoint` pointing only to Harbor forces all image pulls to go through the internal registry, effectively creating a registry sandbox. This leverages the CRI (Container Runtime Interface) mirroring mechanism, which is designed for caching but can be repurposed for strict enforcement by setting the mirror endpoint to the private registry and omitting the upstream registry. In a real-world scenario, this approach is resilient to misconfigured manifests because the runtime intercepts the pull request before any network call to the public registry is made.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Supply Chain Security — study guide chapter
Learn the concepts, then practise the questions
- →
Supply Chain Security practice questions
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Certified Kubernetes Security Specialist CKS study guide
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FAQ
Questions learners often ask
What does this CKS question test?
Supply Chain Security — This question tests Supply Chain Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure the containerd configuration on each node to use Harbor as a mirror for all registries and set endpoint to Harbor only, disabling direct pull from public registries. — Option C is correct because configuring containerd to use Harbor as a mirror for all registries and setting the endpoint to Harbor only effectively blocks pulls from any external registry at the container runtime level. This approach works even if Kubernetes manifests reference Docker Hub images, as the kubelet will redirect all image pull requests to the internal Harbor registry, preventing direct pulls from public registries. It enforces the policy without requiring immediate changes to existing manifests, which aligns with the constraint of not modifying them due to a large backlog.
What should I do if I get this CKS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "immediately / without restart". Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 30, 2026
This CKS practice question is part of Courseiva's free CNCF certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CKS exam.
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