Question 135 of 997

Minimize Microservice Vulnerabilities →mediumMultiple SelectObjective-mapped

Enforcing Read-Only Root Filesystem for Containers — Kubernetes Security | Certified Kubernetes Security Specialist Explained

This CKS practice question tests your understanding of minimize microservice vulnerabilities. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.

Which TWO of the following are valid ways to enforce that containers run with a read-only root filesystem?

A
Setting `runAsNonRoot: true` in the pod's securityContext
Why wrong: This only prevents running as root, not filesystem writability.
B
Using an emptyDir volume mounted at /
Why wrong: Mounting a writable volume over root does not enforce read-only; it actually makes root writable.
C
Setting `fsGroup: 1000` in the pod's securityContext
Why wrong: This sets the group ID for volumes, not read-only root.
D
Using a MutatingWebhookConfiguration that adds `readOnlyRootFilesystem: true` to all containers
A mutating webhook can automatically inject the setting.
E
Setting `readOnlyRootFilesystem: true` in the container's securityContext
This directly sets the filesystem to read-only.

Full breakdown with real-world context →

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

✓

Using a MutatingWebhookConfiguration that adds `readOnlyRootFilesystem: true` to all containers

Option D is correct because a MutatingWebhookConfiguration can intercept Pod creation requests and automatically add the `readOnlyRootFilesystem: true` field to every container's securityContext, enforcing a read-only root filesystem without requiring manual changes to Pod specs. Option E is correct because setting `readOnlyRootFilesystem: true` directly in the container's securityContext is the explicit Kubernetes API field that makes the container's root filesystem read-only, preventing writes to the filesystem layer.

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.

✗
Setting `runAsNonRoot: true` in the pod's securityContext
Why it's wrong here
This only prevents running as root, not filesystem writability.
✗
Using an emptyDir volume mounted at /
Why it's wrong here
Mounting a writable volume over root does not enforce read-only; it actually makes root writable.
✗
Setting `fsGroup: 1000` in the pod's securityContext
Why it's wrong here
This sets the group ID for volumes, not read-only root.
✓
Using a MutatingWebhookConfiguration that adds `readOnlyRootFilesystem: true` to all containers
Why this is correct
A mutating webhook can automatically inject the setting.
Related concept
Read the scenario before looking for a memorised answer.
✓
Setting `readOnlyRootFilesystem: true` in the container's securityContext
Why this is correct
This directly sets the filesystem to read-only.
Related concept
Read the scenario before looking for a memorised answer.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The CKS exam often tests the distinction between Pod-level and container-level securityContext fields, and candidates may incorrectly assume that Pod-level settings like `runAsNonRoot` or `fsGroup` affect the root filesystem's write permissions, when only the container-level `readOnlyRootFilesystem` field (or a mutating webhook) actually enforces that behavior.

Detailed technical explanation

How to think about this question

Under the hood, `readOnlyRootFilesystem: true` works by setting the `readonly` flag on the container's rootfs mount in the container runtime (e.g., runc), which causes any write attempt to the root filesystem to fail with an EROFS error. A MutatingWebhookConfiguration uses an admission webhook that modifies the Pod spec before it is persisted, allowing cluster-wide enforcement without requiring developers to remember the setting. In practice, this is critical for immutable infrastructure patterns where containers should not write to their own filesystem, reducing the attack surface for malware that writes malicious files.

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.

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FAQ

Questions learners often ask

What does this CKS question test?

Minimize Microservice Vulnerabilities — This question tests Minimize Microservice Vulnerabilities — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: Using a MutatingWebhookConfiguration that adds `readOnlyRootFilesystem: true` to all containers — Option D is correct because a MutatingWebhookConfiguration can intercept Pod creation requests and automatically add the `readOnlyRootFilesystem: true` field to every container's securityContext, enforcing a read-only root filesystem without requiring manual changes to Pod specs. Option E is correct because setting `readOnlyRootFilesystem: true` directly in the container's securityContext is the explicit Kubernetes API field that makes the container's root filesystem read-only, preventing writes to the filesystem layer.

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.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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Same concept, more angles

2 more ways this is tested on CKS

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A developer wants to ensure that all containers in a pod run with a read-only root filesystem except for a specific volume mounted for writing logs. Which container-level security context field should be set to true?

medium

A.allowPrivilegeEscalation
✓ B.readOnlyRootFilesystem
C.privileged
D.runAsNonRoot

Why B: Setting `readOnlyRootFilesystem: true` in the container-level security context forces the container's root filesystem to be read-only, preventing any writes to the root filesystem. This is exactly what the developer needs to enforce immutability for the root filesystem while allowing writes only to a specific volume (e.g., for logs) mounted with write access. The field is a boolean in the `securityContext` of a container specification in Kubernetes.

Variation 2. A DevOps engineer wants to ensure that all microservice containers run with a read-only root filesystem to prevent unauthorized writes. What is the simplest way to enforce this at the Pod level?

easy

A.Set `securityContext.runAsNonRoot: true` in the Pod spec
B.Mount an emptyDir volume to the container's writable directories
✓ C.Set `securityContext.readOnlyRootFilesystem: true` in the Pod spec
D.Set `securityContext.privileged: false` in the Pod spec

Why C: Option C is correct because setting `securityContext.readOnlyRootFilesystem: true` in the Pod spec directly enforces that the container's root filesystem is read-only, preventing any unauthorized writes to the root filesystem. This is the simplest and most direct way to achieve the requirement at the Pod level, as it applies to all containers in the Pod unless overridden at the container level.

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