CS0-003 Incident Response and Management Practice Question
After containing a data breach, the incident response team discovers that an attacker exfiltrated sensitive data over DNS tunneling. Which of the following detection rules would BEST identify similar activity in the future?
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
✓
A YARA rule that flags DNS queries with high entropy domain names
DNS tunneling often involves unusual domain names with high entropy or long subdomains. A YARA rule targeting DNS query patterns can detect such anomalies.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
An SIEM alert for any DNS query exceeding 100 bytes
Why it's wrong here
While DNS tunneling often utilizes larger TXT or MX records to exfiltrate data, setting a flat threshold of 100 bytes for all DNS queries will trigger an overwhelming number of false positives. Legitimate queries, such as those for DNSSEC, IPv6 (AAAA), or complex TXT records, frequently exceed this size, making this alert impractical for production environments.
- ✗
A Snort rule blocking traffic to known malicious IPs
Why it's wrong here
Blocking known malicious IP addresses is a reactive, static defense mechanism that fails against DNS tunneling. Because DNS tunneling routes malicious payloads through legitimate, trusted recursive DNS resolvers rather than directly to an attacker-controlled IP, IP-based blocking at the firewall or IDS level will not intercept this traffic.
- ✗
A firewall rule that blocks all DNS requests from internal servers
Why it's wrong here
Implementing a blanket block on all outbound DNS requests from internal servers would severely disrupt critical network operations, including active directory replication, software updates, and basic internet connectivity. Security teams must instead implement selective filtering or forward internal DNS traffic through authorized, monitored internal resolvers rather than blocking DNS entirely.
- ✓
A YARA rule that flags DNS queries with high entropy domain names
Why this is correct
DNS tunneling utilities typically encode stolen data or command-and-control instructions into the subdomain portion of a query, resulting in highly randomized, high-entropy character strings. Using a YARA rule or similar detection mechanism to analyze DNS logs for high-entropy domains allows security analysts to precisely identify anomalous, machine-generated queries indicative of exfiltration without disrupting legitimate network traffic.
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Related to this question
Learn chapter
Regulatory Breach Notification Requirements
Key term
Incident response
Incident response is the structured approach an organization uses to identify, contain, and recover from cybersecurity incidents like data breaches or ransomware attacks.
Key term
YARA
YARA is a pattern-matching tool used by cybersecurity professionals to identify and classify malware based on textual or binary patterns.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CS0-004 practice question is part of Courseiva's free CompTIA 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 CS0-004 exam.