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 to CERT/CC is coordination and disclosure, not containment; it neither stops the intrusion nor limits spread during active response. It is appropriate for notifying authorities about a significant vulnerability. Immediate isolation or blocking of affected systems is what halts exploitation while no patch exists.
- ✗
Rebuild all affected systems
Why it's wrong here
Rebuilding affected systems removes the malware but does not stop the zero-day from re-exploiting freshly rebuilt hosts, since no patch or mitigation exists yet; containment or isolation is needed first. Rebuild is tempting because it eradicates persistent artefacts, and it would be right once a patch or compensating control is available.
- ✗
Apply a vendor patch
Why it's wrong here
No vendor patch exists yet for a zero-day, so nothing can be applied immediately; patching becomes viable only once the vendor releases a fix. The option suits known, patched vulnerabilities. Immediate containment, such as isolation or blocking, addresses the exposure while no patch is available.
- ✓
Implement compensating controls
Why this is correct
No patch exists for a zero-day, so patching cannot remove the exposure. Compensating controls—such as network segmentation, tightened firewall rules or enhanced monitoring—reduce exploitability or impact immediately, containing the threat while the vendor develops a fix. This directly addresses the stem's demand for the best immediate action.
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
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