- A
ValidatingAdmissionWebhook
Why wrong: Validating webhooks would reject the Pod entirely, not cause CrashLoopBackOff.
- B
MutatingAdmissionWebhook
A mutating webhook could modify the Pod spec (e.g., adding a sidecar or changing command) causing the container to fail.
- C
PodSecurity
Why wrong: Would reject the Pod if it violates security standards, not cause restart.
- D
PersistentVolumeClaimResize
Why wrong: Unrelated to Pod startup.
Question 638 of 997
Why a Misconfigured Mutating Webhook Can Cause CrashLoopBackOff
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.
An admin runs 'kubectl run test-pod --image=nginx:latest' and the Pod is created but immediately enters 'CrashLoopBackOff'. 'kubectl describe pod test-pod' shows 'Back-off restarting failed container'. Which admission controller might cause this if misconfigured?
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
✓
MutatingAdmissionWebhook
A MutatingAdmissionWebhook can modify Pod specifications (e.g., injecting sidecar containers, changing image names, or adding init containers) before the Pod is persisted. If the webhook misconfigures the Pod—such as replacing the image with a non-existent one or adding a failing init container—the container may fail to start, causing a CrashLoopBackOff. The 'Back-off restarting failed container' message indicates the container itself is failing, which aligns with a mutation that breaks the Pod's runtime behavior.
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.
- ✗
ValidatingAdmissionWebhook
Why it's wrong here
Validating webhooks would reject the Pod entirely, not cause CrashLoopBackOff.
- ✓
MutatingAdmissionWebhook
Why this is correct
A mutating webhook could modify the Pod spec (e.g., adding a sidecar or changing command) causing the container to fail.
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.
- ✗
PodSecurity
Why it's wrong here
Would reject the Pod if it violates security standards, not cause restart.
- ✗
PersistentVolumeClaimResize
Why it's wrong here
Unrelated to Pod startup.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a Pod entering CrashLoopBackOff must be due to a security policy (PodSecurity) or a validation rejection, but the 'Back-off restarting failed container' message indicates the container ran and failed, which points to a mutation that altered the container's runtime configuration, not a rejection or security constraint.
Detailed technical explanation
How to think about this question
MutatingAdmissionWebhooks are part of the Kubernetes admission chain and are invoked before the object is persisted; they can inject sidecars, modify environment variables, or change the container command. A common misconfiguration is a webhook that unconditionally adds a volume mount or init container that fails (e.g., a missing image or invalid command), causing the main container to never start or to crash. In real-world scenarios, operators often debug such issues by checking the Pod's YAML after mutation (e.g., via 'kubectl get pod test-pod -o yaml') to see what the webhook injected.
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 practitioner preparing for the CKS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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
Targeted practice on this topic area only
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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: MutatingAdmissionWebhook — A MutatingAdmissionWebhook can modify Pod specifications (e.g., injecting sidecar containers, changing image names, or adding init containers) before the Pod is persisted. If the webhook misconfigures the Pod—such as replacing the image with a non-existent one or adding a failing init container—the container may fail to start, causing a CrashLoopBackOff. The 'Back-off restarting failed container' message indicates the container itself is failing, which aligns with a mutation that breaks the Pod's runtime behavior.
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.
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Last reviewed: Jul 4, 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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