CISSP Security Architecture and Engineering Practice Question
This CISSP practice question tests your understanding of security architecture and engineering. 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.
Exhibit
// /etc/pam.d/sshd configuration
#%PAM-1.0
auth required pam_securetty.so
auth required pam_nologin.so
auth include system-auth
account required pam_nologin.so
account include system-auth
password include system-auth
session required pam_loginuid.so
session include system-auth
// /etc/ssh/sshd_config excerpt
PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
ChallengeResponseAuthentication no
UsePAM yes
Refer to the exhibit. A system administrator reports that SSH public key authentication is failing for a non-root user. The user's public key is correctly placed in ~/.ssh/authorized_keys. Which PAM configuration issue is most likely causing the failure?
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 pam_nologin.so module is required, which can deny access if /etc/nologin exists.
The PAM configuration includes 'auth required pam_nologin.so' before the system-auth include. The pam_nologin module checks for the existence of /etc/nologin file; if it exists, only root can login. Since PermitRootLogin is no, root cannot login either, so all logins are denied. This is a common misconfiguration. The issue is that pam_nologin is called before the authentication stack, and it denies all non-root logins if /etc/nologin exists.
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.
✗
PasswordAuthentication is set to no, which conflicts with PAM.
Why it's wrong here
PubkeyAuthentication is yes, so key-based auth should work independently of password auth.
✗
The pam_nologin.so module is listed twice.
Why it's wrong here
Duplication is not necessarily causing failure; it may just be redundant.
✓
The pam_nologin.so module is required, which can deny access if /etc/nologin exists.
Why this is correct
If /etc/nologin exists, pam_nologin denies all non-root logins, but root login is disabled via PermitRootLogin no, so all logins fail.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The pam_securetty.so module is missing for non-root users.
Why it's wrong here
pam_securetty only affects root logins on insecure terminals.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP 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 CISSP ACL questions on filtering logic and placement.
Security Architecture and Engineering — This question tests Security Architecture and Engineering — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The pam_nologin.so module is required, which can deny access if /etc/nologin exists. — The PAM configuration includes 'auth required pam_nologin.so' before the system-auth include. The pam_nologin module checks for the existence of /etc/nologin file; if it exists, only root can login. Since PermitRootLogin is no, root cannot login either, so all logins are denied. This is a common misconfiguration. The issue is that pam_nologin is called before the authentication stack, and it denies all non-root logins if /etc/nologin exists.
What should I do if I get this CISSP 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 CISSP ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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