The answer is to block the IP address at the perimeter firewall. This is the best immediate action because it stops the ongoing brute force attack from an external IP at the network boundary, cutting off all further authentication attempts before they reach the target system, which aligns with the containment phase of incident response. On the CISSP exam, this scenario tests your understanding of the principle that rapid mitigation takes priority over investigation or configuration changes when an active attack is detected; a common trap is to suggest disabling the user account or changing passwords first, which delays containment and may impact legitimate access. Remember the memory tip: “Block first, ask later”—firewall rules are reversible and fast, while account changes are disruptive and slow.
CISSP Security Operations Practice Question
This CISSP practice question tests your understanding of security operations. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 analyst detects repeated failed login attempts from a single external IP address targeting a user account. What is the best IMMEDIATE action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Block the IP address at the perimeter firewall
Blocking the IP address at the perimeter firewall is the best immediate action because it stops the ongoing brute-force attack at the network boundary, preventing further authentication attempts without affecting the legitimate user's access. This aligns with the principle of containment in incident response, prioritizing rapid mitigation over investigation or configuration changes that could delay the response.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Investigate the source IP's history
Why it's wrong here
Should be done but not the first immediate action; blocking takes priority.
✓
Block the IP address at the perimeter firewall
Why this is correct
Immediately stops the attack.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Disable the targeted user account
Why it's wrong here
Disrupts the legitimate user and may not be necessary if the account is not compromised.
✗
Enable account lockout after three failures
Why it's wrong here
This is a preventive control that should already be in place, not an immediate response.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse 'immediate action' with 'long-term fix' and choose to investigate the IP (A) or implement a policy change (D), failing to recognize that containment (B) must come first in the incident response process.
Detailed technical explanation
How to think about this question
In a typical brute-force attack, the attacker sends repeated authentication requests (e.g., over SSH, RDP, or HTTP Basic Auth) from a single IP. Blocking the IP at the firewall using an ACL or a dynamic block rule (e.g., via iptables or a next-gen firewall) immediately terminates the TCP or UDP sessions in progress, as the firewall drops packets from that source. This approach is preferred over rate-limiting or account lockout because it preserves availability for the legitimate user while stopping the attack at Layer 3/4, which is faster than application-layer controls.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A SOC analyst notices unusual lateral movement in the network at 2 AM. The IR playbook dictates: identify and contain (isolate the affected machine), then eradicate (remove the malware), then recover (restore from backup), then document. Skipping containment before eradication risks the attacker regaining access. Questions like this test the sequence and rationale of incident response phases.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Security Operations — This question tests Security Operations — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Block the IP address at the perimeter firewall — Blocking the IP address at the perimeter firewall is the best immediate action because it stops the ongoing brute-force attack at the network boundary, preventing further authentication attempts without affecting the legitimate user's access. This aligns with the principle of containment in incident response, prioritizing rapid mitigation over investigation or configuration changes that could delay the response.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.