Quick Answer
The correct immediate action is to block the IP address at the firewall. This stops an ongoing brute force attack at the network perimeter by denying all traffic from that single external source, using an access control list (ACL) to filter the malicious packets before they reach the server. Blocking at the firewall preserves server availability and internal operations, unlike disabling the account or changing passwords, which would disrupt legitimate users. On the ISC2 Certified in Cybersecurity CC exam, this scenario tests your understanding of first-response incident handling and the principle of least disruption; a common trap is to overcomplicate the response by investigating logs first, but the search intent for “immediate action brute force attack” demands a swift, perimeter-level block. Remember the mnemonic “F.I.R.E.” for a single-source attack: Firewall, Immediately, Rule, Endpoint safe.
This CC 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 analyst notices repeated failed login attempts to a critical server from a single external IP address. Which immediate action should the analyst take?
Enable two-factor authentication.
Why wrong: Important but not an immediate response to an active attack.
Disable the server's network interface.
Why wrong: Would take the server offline unnecessarily.
Block the IP address at the firewall.
Immediate containment of the attack source.
Change the server's IP address.
Why wrong: Temporary and may not stop the attacker from scanning new IP.
Answer choices
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 firewall.
Blocking the IP address at the firewall is the immediate action because it stops the ongoing brute-force attack at the network perimeter without affecting the server's availability or internal operations. Firewall rules can be applied quickly using access control lists (ACLs) to deny traffic from the specific external IP, which is a standard first response to mitigate a single-source attack.
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.
Enable two-factor authentication.
Why it's wrong here
Important but not an immediate response to an active attack.
Disable the server's network interface.
Why it's wrong here
Would take the server offline unnecessarily.
Block the IP address at the firewall.
Why this is correct
Immediate containment of the attack source.
Related concept
Read the scenario before looking for a memorised answer.
Change the server's IP address.
Why it's wrong here
Temporary and may not stop the attacker from scanning new IP.
Common exam traps
ISC2 often tests the distinction between immediate containment actions (like blocking an IP at the firewall) and long-term security improvements (like enabling 2FA), trapping candidates who confuse proactive hardening with reactive incident response.
Detailed technical explanation
Firewall ACLs operate at the network layer (Layer 3) and can be applied statelessly or statefully; a stateless ACL can immediately drop packets from the offending IP using a simple 'deny ip host <attacker_IP> any' rule. In a real-world scenario, the analyst should also consider that the attacker may be using a botnet or spoofed IPs, so blocking a single IP is a temporary measure while further investigation (e.g., checking logs for patterns, implementing rate limiting) is conducted. The immediate action aligns with the NIST incident response framework's containment phase, prioritizing stopping the attack before eradication.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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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: Block the IP address at the firewall. — Blocking the IP address at the firewall is the immediate action because it stops the ongoing brute-force attack at the network perimeter without affecting the server's availability or internal operations. Firewall rules can be applied quickly using access control lists (ACLs) to deny traffic from the specific external IP, which is a standard first response to mitigate a single-source attack.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This CC 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 CC exam.
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