The answer is that the private key file has permissions set to 644 (world-readable). This is correct because SSH key-based authentication enforces strict file permission checks on the private key; if the key is readable by anyone other than the owner, the SSH daemon refuses it to prevent potential key disclosure, logging the 'Authentication refused: bad permissions' error. On the Systems Security Certified Practitioner SSCP exam, this tests your understanding of OpenSSH security defaults and Linux file permission hardening—a common trap is assuming that any readable key will work, when in fact only 600 or 640 are accepted. Remember the memory tip: "SSH keys need to be kept private, so keep them at 600—no world, no group."
SSCP Access Controls Practice Question
This SSCP practice question tests your understanding of access controls. 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 Linux server administrator configures SSH key-based authentication for user 'admin'. The authentication fails with the error 'Authentication refused: bad permissions' in the logs. What is the most likely cause?
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 private key file has permissions set to 644 (world-readable)
Option B is correct because SSH key-based authentication requires the private key file to have restrictive permissions (typically 600 or 640) to prevent unauthorized access. When the private key is world-readable (e.g., 644), the SSH daemon refuses to use it for security reasons, logging 'Authentication refused: bad permissions'. This is a direct check performed by OpenSSH to protect against key disclosure.
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.
✗
The SSH service is not running
Why it's wrong here
If SSH service were down, the connection would be refused entirely, not specific key permission errors.
✓
The private key file has permissions set to 644 (world-readable)
Why this is correct
SSH daemon checks private key permissions; if too permissive, it refuses authentication for security.
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.
✗
The public key is not in the ~/.ssh/authorized_keys file
Why it's wrong here
A missing public key would result in 'Permission denied (publickey)' but not a 'bad permissions' error.
✗
The passphrase on the private key is incorrect
Why it's wrong here
An incorrect passphrase would prompt for a passphrase multiple times or fail with 'Permission denied' but not 'bad permissions'.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between file permission errors and other authentication failures, trapping candidates who confuse 'bad permissions' with missing keys or incorrect passphrases.
Trap categories for this question
Keyword trap
An incorrect passphrase would prompt for a passphrase multiple times or fail with 'Permission denied' but not 'bad permissions'.
Detailed technical explanation
How to think about this question
OpenSSH enforces strict permission checks on the private key file (typically ~/.ssh/id_rsa or ~/.ssh/id_ed25519) and the ~/.ssh directory itself. The daemon checks that the private key is not accessible by group or others (mode 600 or 640 is allowed, but 644 is not). This behavior is defined in the sshd_config option 'StrictModes', which is enabled by default. In real-world scenarios, this often occurs when keys are copied with default umask settings or extracted from archives without preserving permissions.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
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.
Access Controls — This question tests Access Controls — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The private key file has permissions set to 644 (world-readable) — Option B is correct because SSH key-based authentication requires the private key file to have restrictive permissions (typically 600 or 640) to prevent unauthorized access. When the private key is world-readable (e.g., 644), the SSH daemon refuses to use it for security reasons, logging 'Authentication refused: bad permissions'. This is a direct check performed by OpenSSH to protect against key disclosure.
What should I do if I get this SSCP 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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