Quick Answer
The answer is to set PermitRootLogin to no, which is one of three essential SSH configurations to prevent unauthorized access. This directive in /etc/ssh/sshd_config disables direct root login via SSH, forcing administrators to authenticate as a regular user and then use sudo or su for elevated commands. By eliminating the root account as an attack vector, you block brute-force attempts targeting the most privileged user, even if an attacker guesses the password. On the CompTIA Linux+ XK0-005 exam, this topic tests your ability to harden SSH against common threats; a frequent trap is confusing PermitRootLogin with AllowUsers, which whitelists specific accounts instead. Remember the memory tip: “No root, no brute” — disabling root login stops the most targeted credential attack, while combining it with AllowUsers and key-based authentication creates a layered defense that the exam rewards.
This XK0-005 practice question tests your understanding of security. 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.
Which THREE of the following actions can help prevent unauthorized access to a Linux server via SSH?
Allow only specific users with AllowUsers.
Restricts SSH access to authorized users.
Set MaxAuthTries to 6.
Why wrong: Default is already 6, does not prevent brute force effectively.
Use protocol version 1.
Why wrong: Version 1 has known vulnerabilities.
Disable password authentication.
Forces key-based authentication, stronger.
Set PermitRootLogin to no.
Prevents direct root login.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Allow only specific users with AllowUsers.
Option A is correct because the `AllowUsers` directive in `/etc/ssh/sshd_config` restricts SSH logins to only the specified user accounts, blocking all others even if they have valid credentials. This reduces the attack surface by explicitly whitelisting authorized users, making it an effective access control measure.
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.
Allow only specific users with AllowUsers.
Why this is correct
Restricts SSH access to authorized users.
Related concept
Read the scenario before looking for a memorised answer.
Set MaxAuthTries to 6.
Why it's wrong here
Default is already 6, does not prevent brute force effectively.
Use protocol version 1.
Why it's wrong here
Version 1 has known vulnerabilities.
Disable password authentication.
Why this is correct
Forces key-based authentication, stronger.
Related concept
Read the scenario before looking for a memorised answer.
Set PermitRootLogin to no.
Why this is correct
Prevents direct root login.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
CompTIA often tests the misconception that increasing `MaxAuthTries` (option B) or using protocol version 1 (option C) improves security, when in fact they either have no preventive effect or actively weaken security.
Detailed technical explanation
Under the hood, `PermitRootLogin no` (option E) prevents direct root login via SSH, forcing attackers to first compromise a regular user account and then escalate privileges, adding a layer of defense. Disabling password authentication (option D) forces the use of public key authentication, which is resistant to brute-force attacks because the private key is never transmitted and requires possession of the key file. In real-world scenarios, combining `AllowUsers` with key-only authentication and disabling root login is a standard hardening practice recommended by benchmarks like CIS and DISA STIG.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
Security — This question tests Security — Read the scenario before looking for a memorised answer..
The correct answer is: Allow only specific users with AllowUsers. — Option A is correct because the `AllowUsers` directive in `/etc/ssh/sshd_config` restricts SSH logins to only the specified user accounts, blocking all others even if they have valid credentials. This reduces the attack surface by explicitly whitelisting authorized users, making it an effective access control measure.
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 XK0-005 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 XK0-005 exam.
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