Scenario: A user receives “authentication failure” on login. Question: What should you check first?

Question

BlackTom AI · Accepted Answer

Authentication failure indicates a problem with credentials or the account, so check password expiry and account status first.

1) Relevant knowledge points and concepts
- Authentication failures are usually caused by incorrect credentials, expired passwords, or locked/disabled accounts.
- System resources (CPU, disk) and file permissions affect service availability but rarely produce an “authentication failure” message.
- Useful diagnostic sources: authentication logs (e.g., /var/log/auth.log, /var/log/secure, Windows Security Event Log), account status commands (passwd -S, chage -l on Linux; AD Users and Computers or net user on Windows).

2) Complete reasoning process (step-by-step)
- Step 1: Confirm user entered the correct username and password (check for Caps Lock, keyboard layout).
- Step 2: Check account status: is the account locked, disabled, or expired? On Linux use passwd -S username or chage -l username; on Windows check account expiration/lockout in AD or run net user username.
- Step 3: Inspect authentication logs for explicit messages (e.g., “Account expired”, “Authentication token expired”, “Account locked due to too many failed attempts”).
- Step 4: If account is okay, verify upstream authentication services (LDAP/AD, RADIUS) are reachable and responding.
- Step 5: If still unresolved, consider system-level issues (service down, disk full preventing logging) as secondary causes.

3) Intermediate steps and checks (commands/logs)
- Linux: sudo passwd -S alice ; sudo chage -l alice ; sudo tail -n 50 /var/log/auth.log
- Windows: Open Event Viewer > Windows Logs > Security; AD Users and Computers > user properties > Account; net user alice
These quickly reveal expiry, lockouts, or disabled accounts.

4) Why the correct answer is correct
- The error text “authentication failure” most directly maps to credential/account problems. Checking password expiry/account status targets the most likely root cause first, quickly resolving common issues like expired passwords or locked accounts.

5) Why other options are incorrect
- a. CPU usage: high CPU can slow authentication but would not typically return an “authentication failure” message.
- b. File permissions: relevant for service operation or SSH keys but not the primary cause of a generic authentication failure for normal username/password login.
- c. Disk space: a full disk can break services or logging, but it’s a less likely immediate cause and should be checked only after verifying account/auth issues.

6) Conclusion
Checking password expiry and account status is the correct first action; this matches the provided answer: "Password expiry and account status."

Scenario: A user receives “authentication failure” on login. Question: What should you check first?单项选择题

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