This EX200 practice question tests your understanding of manage security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 user reports that SSH key-based authentication fails, but password authentication works. The admin checks /etc/ssh/sshd_config: PubkeyAuthentication yes, PasswordAuthentication no (contrary to the report). Which is the most likely reason key-based auth fails?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The .ssh directory has permissions 755.
Option D is correct because SSH key-based authentication requires that the ~/.ssh directory and its contents have restrictive permissions. If the .ssh directory has permissions 755, it is world-readable, which violates SSH's security model. SSH daemon (sshd) will refuse to use the authorized_keys file if the directory permissions are too permissive, causing key-based authentication to fail even though PubkeyAuthentication is enabled.
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.
✗
SELinux is blocking SSH from reading the key.
Why it's wrong here
SELinux context for authorized_keys is typically correct; this is a common misconception.
✗
The public key is not in ~/.ssh/authorized_keys.
Why it's wrong here
That would cause failure, but the question implies password auth works, so likely key is there but permissions wrong.
✗
SSH is configured to use a different port.
Why it's wrong here
Port change does not affect key authentication.
✓
The .ssh directory has permissions 755.
Why this is correct
sshd requires .ssh to be 700; 755 is too permissive.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume key-based authentication fails only due to missing keys or disabled PubkeyAuthentication, overlooking the strict permission requirements that SSH enforces for security.
Detailed technical explanation
How to think about this question
SSH enforces strict permission checks on ~/.ssh/authorized_keys and the ~/.ssh directory to prevent unauthorized users from injecting keys. The required permissions are 700 for ~/.ssh and 600 for authorized_keys; any deviation (e.g., 755) causes sshd to ignore the file and fall back to other authentication methods if available. This behavior is defined in the OpenSSH source code and documented in the sshd_config manual, ensuring that only the key owner can modify authentication credentials.
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 practitioner preparing for the EX200 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Manage security — This question tests Manage security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The .ssh directory has permissions 755. — Option D is correct because SSH key-based authentication requires that the ~/.ssh directory and its contents have restrictive permissions. If the .ssh directory has permissions 755, it is world-readable, which violates SSH's security model. SSH daemon (sshd) will refuse to use the authorized_keys file if the directory permissions are too permissive, causing key-based authentication to fail even though PubkeyAuthentication is enabled.
What should I do if I get this EX200 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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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