Quick Answer
The correct first action during a DDoS attack that is saturating an internet link is to activate cloud-based DDoS mitigation services. This is because cloud-based mitigation platforms are specifically designed to absorb and filter massive volumes of malicious traffic before it reaches the organization’s own network infrastructure, preserving legitimate access while the attack is ongoing. On the Certified Information Security Manager CISM exam, this scenario tests your understanding of incident response prioritization under the Domain 4: Information Security Incident Management; the common trap is choosing rate limiting or null-routing, but rate limiting fails under saturation and null-routing blocks all traffic, including valid users. A strong memory tip is to think of the cloud as a “firewall in the sky” that scales instantly—when the pipe is full, you need an off-ramp, not a narrower lane.
This CISM practice question tests your understanding of incident management. 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.
An organization is under a DDoS attack that is saturating their internet link. The incident response team needs to mitigate the attack. Which action should be taken first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Activate cloud-based DDoS mitigation services.
Correct: Scalable and effective.
Shut down all public-facing services.
Why wrong: Incorrect: Drastic and impacts business.
Implement rate limiting on the perimeter firewall.
Why wrong: Incorrect: May not handle volumetric attacks.
Contact the ISP to null-route the attack IPs.
Why wrong: Incorrect: Null-routing also blocks legitimate traffic.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Activate cloud-based DDoS mitigation services.
Option D is correct because cloud-based DDoS mitigation services are designed to absorb large-scale attacks. Rate limiting may be insufficient; null-routing can block legitimate traffic; shutting down services is a last resort.
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.
Activate cloud-based DDoS mitigation services.
Why this is correct
Correct: Scalable and effective.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Shut down all public-facing services.
Why it's wrong here
Incorrect: Drastic and impacts business.
Implement rate limiting on the perimeter firewall.
Why it's wrong here
Incorrect: May not handle volumetric attacks.
Contact the ISP to null-route the attack IPs.
Why it's wrong here
Incorrect: Null-routing also blocks legitimate traffic.
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 CISM 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 CISM 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.
Incident Management — study guide chapter
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FAQ
Incident Management — This question tests Incident Management — Read the scenario before looking for a memorised answer..
The correct answer is: Activate cloud-based DDoS mitigation services. — Option D is correct because cloud-based DDoS mitigation services are designed to absorb large-scale attacks. Rate limiting may be insufficient; null-routing can block legitimate traffic; shutting down services is a last resort.
Identify which CISM 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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This CISM practice question is part of Courseiva's free ISACA 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 CISM exam.
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