The answer is the line account required pam_nologin.so. This PAM module directly causes new SSH sessions to fail for all users because it checks for the existence of the /etc/nologin file; if that file is present, the module denies login to everyone except root, making it a common cause of pam_nologin login failures in Linux. On the CompTIA Linux+ XK0-005 exam, this scenario tests your understanding of PAM stack ordering and the specific behavior of the pam_nologin module, often appearing as a trap where you must distinguish it from other modules like pam_securetty or pam_access. A key memory tip: think of /etc/nologin as a “do not disturb” sign for the system—if it exists, pam_nologin slams the door on all non-root logins, so when you see “all users fail,” immediately suspect that file and this PAM line.
XK0-005 Security Practice Question
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.
Exhibit
Refer to the exhibit.
/etc/pam.d/sshd:
auth required pam_sepermit.so
auth substack password-auth
auth include postlogin
account required pam_nologin.so
account include password-auth
password include password-auth
session required pam_selinux.so close
session required pam_loginuid.so
session required pam_selinux.so open
session required pam_namespace.so
session optional pam_keyinit.so force revoke
session include password-auth
session include postlogin
An administrator notices that new SSH sessions fail for all users. Which line in the exhibit 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.
Refer to the exhibit.
/etc/pam.d/sshd:
auth required pam_sepermit.so
auth substack password-auth
auth include postlogin
account required pam_nologin.so
account include password-auth
password include password-auth
session required pam_selinux.so close
session required pam_loginuid.so
session required pam_selinux.so open
session required pam_namespace.so
session optional pam_keyinit.so force revoke
session include password-auth
session include postlogin
A
account required pam_nologin.so
If /etc/nologin exists, this module denies login to all non-root users.
B
session include password-auth
Why wrong: This includes default password authentication session rules, typically not causing failures.
C
auth required pam_sepermit.so
Why wrong: This is for SELinux and typically not the cause of total login failure.
D
session required pam_loginuid.so
Why wrong: This sets the loginuid process attribute and is standard.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
account required pam_nologin.so
The line 'account required pam_nologin.so' causes new SSH sessions to fail because the pam_nologin module checks for the existence of /etc/nologin. If that file exists, it denies login to all non-root users. This is commonly used during maintenance to prevent new logins, and since the question states 'all users' (including root if root is not explicitly exempted), this PAM module is the direct cause of the failure.
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.
✓
account required pam_nologin.so
Why this is correct
If /etc/nologin exists, this module denies login to all non-root users.
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.
✗
session include password-auth
Why it's wrong here
This includes default password authentication session rules, typically not causing failures.
✗
auth required pam_sepermit.so
Why it's wrong here
This is for SELinux and typically not the cause of total login failure.
✗
session required pam_loginuid.so
Why it's wrong here
This sets the loginuid process attribute and is standard.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the 'account' stack with the 'auth' or 'session' stacks, mistakenly thinking a missing authentication module (like pam_sepermit.so) or a session module (like pam_loginuid.so) is the cause, when in fact the account-level pam_nologin.so is specifically designed to block new logins system-wide.
Detailed technical explanation
How to think about this question
The pam_nologin module is typically placed in the 'account' stack of PAM configuration files (e.g., /etc/pam.d/sshd or /etc/pam.d/login). When /etc/nologin exists, the module returns PAM_AUTH_ERR for all users except root (unless the 'nodelay' or 'no_warn' options are used). This is a common administrative tool during system maintenance or emergency shutdowns, and the file's contents are often displayed to the user as a message before the connection is dropped.
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 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 XK0-005 question in full detail.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: account required pam_nologin.so — The line 'account required pam_nologin.so' causes new SSH sessions to fail because the pam_nologin module checks for the existence of /etc/nologin. If that file exists, it denies login to all non-root users. This is commonly used during maintenance to prevent new logins, and since the question states 'all users' (including root if root is not explicitly exempted), this PAM module is the direct cause of the failure.
What should I do if I get this XK0-005 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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