The answer is that the client's system clock is not synchronized with the domain controller. Kerberos clock skew is the most likely cause of this authentication failure because the protocol relies on timestamps within tickets to prevent replay attacks; if the client’s time differs from the Key Distribution Center (KDC) by more than the default tolerance—typically five minutes in Windows Active Directory—the KDC rejects the request outright. On the CISSP exam, this concept tests your understanding of Kerberos’s dependency on synchronized time as a fundamental security control, often appearing in questions about authentication protocols or replay attack prevention. A common trap is confusing this with a password issue or expired ticket, but remember: a complete inability to authenticate, rather than a gradual timeout, points directly to clock drift. Memory tip: “Five minutes off, five times the trouble”—if the clock skew exceeds the default window, authentication fails immediately.
CISSP Identity and Access Management Practice Question
This CISSP practice question tests your understanding of identity and access management. 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.
Refer to the exhibit. A user is unable to authenticate using Kerberos. 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 client's system clock is not synchronized with the domain controller.
Kerberos relies heavily on time synchronization between the client and the Key Distribution Center (KDC) because it uses timestamps to prevent replay attacks. If the client's system clock differs from the domain controller's clock by more than the default maximum tolerance (typically 5 minutes in Windows Active Directory), the KDC will reject the authentication request, resulting in failure. This is the most likely cause because the user is unable to authenticate at all, not just after a period of inactivity.
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 Kerberos ticket-granting ticket (TGT) has expired.
Why it's wrong here
Expired TGT would show ticket expiration, not pre-authentication failure.
✗
The user's account is locked out.
Why it's wrong here
Lockout would be indicated by a different event or status.
✓
The client's system clock is not synchronized with the domain controller.
Why this is correct
Time offset causes pre-authentication to fail.
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 user's password has expired.
Why it's wrong here
Expired password typically gives a different error code.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume Kerberos failures are always due to credential issues (expired passwords or locked accounts), but the protocol's strict time synchronization requirement is a classic and frequently tested cause of authentication failures.
Trap categories for this question
Command / output trap
Expired TGT would show ticket expiration, not pre-authentication failure.
Detailed technical explanation
How to think about this question
Kerberos uses the AS-REQ and AS-REP messages during initial authentication, where the client sends a timestamp encrypted with the user's long-term key. The KDC decrypts this timestamp and compares it to its own clock; if the difference exceeds the configured skew (default 5 minutes in Windows, configurable via Kerberos policy), the KDC returns a KRB_AP_ERR_SKEW error. In real-world scenarios, this often occurs after daylight saving time changes or when a VM's clock drifts due to host clock issues.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Identity and Access Management — This question tests Identity and Access Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The client's system clock is not synchronized with the domain controller. — Kerberos relies heavily on time synchronization between the client and the Key Distribution Center (KDC) because it uses timestamps to prevent replay attacks. If the client's system clock differs from the domain controller's clock by more than the default maximum tolerance (typically 5 minutes in Windows Active Directory), the KDC will reject the authentication request, resulting in failure. This is the most likely cause because the user is unable to authenticate at all, not just after a period of inactivity.
What should I do if I get this CISSP 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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