The answer is to edit /etc/ssh/sshd_config to set PermitRootLogin no and PasswordAuthentication no, then restart sshd. This is correct because the log entry shows a failed password attempt for root from an external IP, indicating an active brute-force or password guessing attack. Disabling root login eliminates the highest-value target, while disabling password authentication forces the use of SSH keys, which are resistant to such attacks. On the CHFI exam, this scenario tests your ability to perform an immediate response to a Linux SSH failed login incident, distinguishing between forensic analysis and containment actions. A common trap is to focus on blocking the single IP (e.g., with iptables) rather than hardening the SSH configuration, which prevents all future root password attacks. Memory tip: “No root, no password” – the two immediate toggles for SSH security.
CHFI OS and Network Forensics Practice Question
This CHFI practice question tests your understanding of os and network forensics. 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.
During an incident response on a Linux server, you find the following entry in /var/log/auth.log: "Mar 10 12:34:56 server sshd[1234]: Failed password for root from 10.0.0.5 port 34567 ssh2". Which of the following is the BEST immediate action to prevent further unauthorized access?
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
✓
Edit /etc/ssh/sshd_config to set PermitRootLogin no and PasswordAuthentication no, then restart sshd
The log shows a failed SSH login attempt for root from an external IP. The most effective immediate action is to change the SSH configuration to disable root login and key-based authentication only (PasswordAuthentication no), then restart the service. This blocks password guessing attacks on root.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Block the IP address 10.0.0.5 at the firewall
Why it's wrong here
Blocking one IP is temporary; the attacker can use other IPs. Better to harden SSH configuration.
✓
Edit /etc/ssh/sshd_config to set PermitRootLogin no and PasswordAuthentication no, then restart sshd
Why this is correct
This disables root SSH login and password authentication, forcing key-based authentication, which mitigates brute-force attacks.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
Delete the root user account
Why it's wrong here
Deleting root is extreme and breaks system administration. Disabling root SSH login is sufficient.
✗
Change the root password to a complex password
Why it's wrong here
Changing root password helps but does not prevent password guessing attacks. Disabling root SSH login is more effective.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CHFI question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CHFI ACL questions on filtering logic and placement.
OS and Network Forensics — This question tests OS and Network Forensics — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Edit /etc/ssh/sshd_config to set PermitRootLogin no and PasswordAuthentication no, then restart sshd — The log shows a failed SSH login attempt for root from an external IP. The most effective immediate action is to change the SSH configuration to disable root login and key-based authentication only (PasswordAuthentication no), then restart the service. This blocks password guessing attacks on root.
What should I do if I get this CHFI question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CHFI ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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