Question 609 of 997

Supply Chain Security →mediumMultiple SelectObjective-mapped

Dockerfile Best Practice: Use a Non-Root User (RUN adduser && USER)

This CKS practice question tests your understanding of supply chain security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 best practices for Dockerfile security according to CKS guidelines?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.

A
RUN useradd -m myuser && USER root
Why wrong: Switching back to root negates the benefit of creating a non-root user.
B
COPY --from=builder /app /app
Why wrong: While multi-stage builds can reduce image size, this specific line is part of multi-stage and not a standalone security best practice. The question asks for 'best practices for Dockerfile security', and multi-stage is more about build optimization.
C
RUN adduser -D myuser && USER myuser
This creates a non-root user and switches to it, following security best practice.
D
FROM scratch
Using a minimal base image like scratch reduces the attack surface.
E
FROM alpine:latest
Why wrong: Using 'latest' tag is not reproducible and may introduce unexpected changes.

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

✓

RUN adduser -D myuser && USER myuser

Option C is correct because it creates a dedicated non-root user (`adduser -D myuser`) and then switches to that user with `USER myuser`, ensuring the container runs without root privileges. This aligns with the CKS best practice of least privilege, reducing the risk of privilege escalation if the container is compromised. The `-D` flag in Alpine's `adduser` creates a user without a password, which is appropriate for containerized environments.

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.

✗
RUN useradd -m myuser && USER root
Why it's wrong here
Switching back to root negates the benefit of creating a non-root user.
✗
COPY --from=builder /app /app
Why it's wrong here
While multi-stage builds can reduce image size, this specific line is part of multi-stage and not a standalone security best practice. The question asks for 'best practices for Dockerfile security', and multi-stage is more about build optimization.
✓
RUN adduser -D myuser && USER myuser
Why this is correct
This creates a non-root user and switches to it, following security best practice.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✓
FROM scratch
Why this is correct
Using a minimal base image like scratch reduces the attack surface.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
FROM alpine:latest
Why it's wrong here
Using 'latest' tag is not reproducible and may introduce unexpected changes.

Common exam traps

Common exam trap: answer the scenario, not the keyword

CKS often tests the misconception that simply creating a user (without switching to it) or using multi-stage builds is sufficient for security, when the key is actually ensuring the container process runs as a non-root user.

Detailed technical explanation

How to think about this question

Under the hood, containers share the host kernel, so a root user inside the container has capabilities that can be abused for container escape (e.g., via `CAP_SYS_ADMIN`). The `USER` directive in a Dockerfile sets the user for subsequent `RUN`, `CMD`, and `ENTRYPOINT` instructions, but it does not drop all capabilities; combining it with `--cap-drop=ALL` in the runtime further hardens the container. In a real-world scenario, a web application running as root could allow an attacker who exploits a remote code execution vulnerability to gain full control of the container and potentially the host.

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.

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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: RUN adduser -D myuser && USER myuser — Option C is correct because it creates a dedicated non-root user (`adduser -D myuser`) and then switches to that user with `USER myuser`, ensuring the container runs without root privileges. This aligns with the CKS best practice of least privilege, reducing the risk of privilege escalation if the container is compromised. The `-D` flag in Alpine's `adduser` creates a user without a password, which is appropriate for containerized environments.

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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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