Quick Answer
The answer is to implement compensating controls. When a zero-day vulnerability is the root cause, no vendor patch exists, so the immediate action for zero-day vulnerability must focus on reducing risk through temporary measures like firewall rules, intrusion detection signatures, or application-layer filtering. This approach aligns with incident response containment, which prioritizes mitigating harm while preserving forensic evidence—rebuilding systems without addressing the flaw only re-exposes them. On the ISC2 Certified in Cybersecurity CC exam, this question tests your understanding of containment strategies under the Incident Response domain; a common trap is choosing “rebuild systems” or “apply a patch,” but since no patch exists for a zero-day, compensating controls are the only viable immediate action. Memory tip: “No patch? No problem—compensate to contain.”
This CC practice question tests your understanding of business continuity, dr & incident response. 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 an incident, the IR team identifies that the root cause is a zero-day vulnerability. Which of the following is the best immediate action?
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.
Report to CERT/CC
Why wrong: Reporting is important but not an immediate mitigation action.
Rebuild all affected systems
Why wrong: Rebuilding is drastic and may not be immediately necessary; compensating controls provide interim protection.
Apply a vendor patch
Why wrong: No patch exists for a zero-day vulnerability.
Implement compensating controls
Compensating controls reduce risk by blocking or detecting exploitation of the vulnerability.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Implement compensating controls
When a zero-day vulnerability is the root cause, no vendor patch exists yet (option C is impossible). Rebuilding systems (option B) without addressing the vulnerability leaves them re-exposed. The best immediate action is to implement compensating controls—such as firewall rules, IDS/IPS signatures, or application-layer filtering—to mitigate the risk until a permanent fix is available. This aligns with incident response containment strategies that prioritize reducing impact while preserving forensic evidence.
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.
Report to CERT/CC
Why it's wrong here
Reporting is important but not an immediate mitigation action.
Rebuild all affected systems
Why it's wrong here
Rebuilding is drastic and may not be immediately necessary; compensating controls provide interim protection.
Apply a vendor patch
Why it's wrong here
No patch exists for a zero-day vulnerability.
Implement compensating controls
Why this is correct
Compensating controls reduce risk by blocking or detecting exploitation of the vulnerability.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
ISC2 often tests the misconception that 'rebuilding systems' or 'applying a patch' are immediate actions for a zero-day, when in reality the absence of a patch and the need for containment make compensating controls the only viable first step.
Detailed technical explanation
Compensating controls for a zero-day often involve deploying virtual patching via a web application firewall (WAF) or network-based intrusion prevention system (NIPS) using custom signatures that block the exploit pattern. For example, if the zero-day is a buffer overflow in an HTTP header, a WAF rule can drop packets matching the specific payload length or character sequence. This approach buys time for the vendor to develop an official patch while maintaining business operations, though it requires careful tuning to avoid false positives.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A security team runs a vulnerability scan on a web application and discovers an unpatched SQL injection flaw. The team prioritises remediation by CVSS score — critical flaws are patched within 24 hours, high within 7 days. Questions like this test whether you understand vulnerability management processes, scanning tools, and remediation prioritisation.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Business Continuity, DR & Incident Response — study guide chapter
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FAQ
Business Continuity, DR & Incident Response — This question tests Business Continuity, DR & Incident Response — Read the scenario before looking for a memorised answer..
The correct answer is: Implement compensating controls — When a zero-day vulnerability is the root cause, no vendor patch exists yet (option C is impossible). Rebuilding systems (option B) without addressing the vulnerability leaves them re-exposed. The best immediate action is to implement compensating controls—such as firewall rules, IDS/IPS signatures, or application-layer filtering—to mitigate the risk until a permanent fix is available. This aligns with incident response containment strategies that prioritize reducing impact while preserving forensic evidence.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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