easyMultiple Choice
ISC2 CC Practice Question: Is configuring a network intrusion detection…
A security engineer is configuring a network intrusion detection system (NIDS) to monitor traffic on a critical subnet. To minimize false positives, which of the following should the engineer baseline first?
⚠ Common exam trap
ISC2 often tests the distinction between anomaly-based and signature-based detection, and the trap here is that candidates mistakenly think vulnerability scans or firewall logs provide a sufficient baseline, when in fact only observed normal traffic patterns during representative periods (like peak hours) can minimize false positives in an anomaly-based NIDS.
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
✓
The normal traffic patterns during peak business hours
Baselining normal traffic patterns during peak business hours establishes a reference of legitimate network behavior, which is essential for a NIDS to distinguish benign anomalies from actual threats. Without this baseline, the NIDS may generate false positives by flagging legitimate peak-hour traffic spikes as malicious. This aligns with the principle that anomaly-based detection relies on a statistical model of normal activity to reduce noise.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The results of a recent vulnerability scan
Why it's wrong here
A vulnerability scan reports software weaknesses, not the normal traffic patterns a NIDS needs to distinguish benign from malicious flows. Baselining that scan would not reduce false positives. Vulnerability scan results would be correct input when prioritising patch remediation, not for tuning intrusion detection signatures.
- ✓
The normal traffic patterns during peak business hours
Why this is correct
Baselining normal peak-hour traffic patterns establishes what legitimate activity looks like, so the NIDS can flag only deviations. This directly satisfies the stem's constraint of minimising false positives, since signatures tuned to a known baseline avoid alerting on routine business traffic.
- ✗
The latest attack signatures from the vendor
Why it's wrong here
Attack signatures define what the NIDS detects, not what normal traffic looks like, so loading them cannot establish a baseline. Signature tuning is tempting because it directly reduces known detections, and would be the right focus when suppressing a specific documented exploit rather than learning the subnet's legitimate traffic patterns.
- ✗
The firewall logs from the past 24 hours
Why it's wrong here
Twenty-four hours of firewall logs capture only permitted and denied flows at the perimeter, not the full packet-level behaviour of the subnet, so they cannot establish a normal-traffic baseline. Firewall logs are tempting because they are readily available, and would suit investigating a specific blocked connection.
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Business Continuity and Disaster Recovery
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using subnet masks.
Key term
Baseline
A baseline is a documented starting point for the normal performance and behavior of a system, network, or component, used to detect changes and troubleshoot issues.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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