Quick Answer
The answer is writing a cron job to the container's writable /tmp directory. This is the most likely persistence mechanism because even when a container enforces a read-only root filesystem, the /tmp directory is almost always mounted as a separate, writable volume to support temporary application data. An attacker who gains a shell can drop a malicious script into /tmp and schedule it via cron, achieving persistence without needing to modify the immutable root filesystem. On the CompTIA SecurityX CAS-004 exam, this question tests your understanding of container isolation boundaries and the common misconfiguration of leaving /tmp writable—a trap where candidates assume a read-only filesystem blocks all writes. Remember the memory tip: "Read-only root, but /tmp is the loot."
This CAS-004 practice question tests your understanding of scripting, containers and automation. 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.
During a red team exercise, an attacker exploits a vulnerability in a containerized web application to gain a shell. The container is running with a read-only root filesystem. What is the most likely persistence mechanism the attacker will use?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Modify the container's entrypoint script in the image.
Why wrong: The image is immutable; changes would not persist.
Write a cron job to the container's writable /tmp directory.
/tmp is often writable and can be used for persistence.
Create a new user account in /etc/passwd.
Why wrong: /etc is typically read-only in the container.
Install a kernel module on the host.
Why wrong: The attacker does not have host access.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Write a cron job to the container's writable /tmp directory.
Option A is correct because /tmp is typically writable even with a read-only root filesystem. Option B and C require writable /etc or image modification. Option D requires host access, which is not given.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Modify the container's entrypoint script in the image.
Why it's wrong here
The image is immutable; changes would not persist.
Write a cron job to the container's writable /tmp directory.
Why this is correct
/tmp is often writable and can be used for persistence.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Create a new user account in /etc/passwd.
Why it's wrong here
/etc is typically read-only in the container.
Install a kernel module on the host.
Why it's wrong here
The attacker does not have host access.
Common exam traps
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the CAS-004 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
Identify which CAS-004 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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FAQ
Scripting, Containers and Automation — This question tests Scripting, Containers and Automation — Read the scenario before looking for a memorised answer..
The correct answer is: Write a cron job to the container's writable /tmp directory. — Option A is correct because /tmp is typically writable even with a read-only root filesystem. Option B and C require writable /etc or image modification. Option D requires host access, which is not given.
Identify which CAS-004 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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