ISC2 CC Business Continuity, DR & Incident Response Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
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.
Go deeper
Related to this question
Learn chapter
Risk Management and Security Controls
Key term
Containment
Containment is the incident response phase where security teams isolate a compromised system or network to prevent the threat from spreading further while preserving evidence.
Key term
IDS
An IDS is a security system that monitors network or system traffic for suspicious activity and alerts administrators to potential threats, but does not actively block them.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CC practice question is part of Courseiva's free ISC2 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 CC exam.