Quick Answer
The answer is port scanning. This is the most likely attack because port scanning from firewall logs appears as a pattern of dropped packets from a single external IP targeting multiple ports on a host, as the firewall blocks unsolicited inbound traffic to closed or filtered ports, which is a classic reconnaissance technique used to map open services. On the Certified Information Systems Professional CISSP exam, this question tests your understanding of network security monitoring and the difference between active reconnaissance and actual exploitation; a common trap is confusing port scanning with a denial-of-service attack, but port scanning typically involves sequential or randomized port probes without overwhelming bandwidth. Remember the mnemonic “DROP” for Detection of Reconnaissance via Odd Ports—if you see many dropped packets from one IP across diverse ports, think scanning, not flooding.
This CISSP practice question tests your understanding of security operations. 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.
A security team is reviewing firewall logs and sees many dropped packets from an external IP. What type of attack is most likely?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Man-in-the-middle
Why wrong: MITM involves intercepting traffic, not being dropped.
Port scanning
Multiple dropped packets suggest scanning for open ports.
SQL injection
Why wrong: SQL injection targets application layer, not firewall drops.
Phishing
Why wrong: Phishing is social engineering, not reflected in firewall drops.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Port scanning
Port scanning is the most likely attack because it involves an external IP sending packets to multiple ports on a target system to identify open services. Firewalls log these as dropped packets when they block unsolicited inbound traffic to closed or filtered ports, which is a common signature of reconnaissance activity.
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.
Man-in-the-middle
Why it's wrong here
MITM involves intercepting traffic, not being dropped.
Port scanning
Why this is correct
Multiple dropped packets suggest scanning for open ports.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
SQL injection
Why it's wrong here
SQL injection targets application layer, not firewall drops.
Phishing
Why it's wrong here
Phishing is social engineering, not reflected in firewall drops.
Common exam traps
The trap here is that candidates may confuse port scanning with a denial-of-service (DoS) attack, but port scanning is reconnaissance, not resource exhaustion, and the key clue is the pattern of dropped packets to multiple ports from a single IP.
Detailed technical explanation
Port scanning often uses TCP SYN scans (half-open scans) to avoid completing the three-way handshake, but firewalls still log dropped SYN packets to closed ports. Tools like Nmap can send packets with various flags (SYN, ACK, FIN) to evade detection, but a firewall rule set to drop all inbound traffic except to specific ports will log these as dropped packets regardless of the scan type. In real-world scenarios, a sudden spike in dropped packets from a single IP across a range of ports (e.g., 1-1024) is a strong indicator of a horizontal port scan.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
An employee at a financial services firm receives an email that appears to come from the IT helpdesk, asking them to reset their password via a link. The link leads to a convincing fake portal that harvests credentials. Security teams use phishing simulations and security-awareness training to reduce this attack vector. Questions like this test whether you can identify social engineering techniques and appropriate controls.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Security Operations — This question tests Security Operations — Read the scenario before looking for a memorised answer..
The correct answer is: Port scanning — Port scanning is the most likely attack because it involves an external IP sending packets to multiple ports on a target system to identify open services. Firewalls log these as dropped packets when they block unsolicited inbound traffic to closed or filtered ports, which is a common signature of reconnaissance activity.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This CISSP 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 CISSP exam.
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