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CISSP Practice Question: A large enterprise uses Active Directory for…
A large enterprise uses Active Directory for authentication. Several users report intermittent authentication failures when accessing internal web applications. The help desk confirms that the failures occur at random times and affect both new and existing users. The security team discovers that the system clocks on domain controllers are within acceptable limits, but some client workstations show time drift of up to 10 minutes. The Kerberos protocol is used for authentication. What is the most likely cause of the authentication failures, and what action should be taken?
⚠ Common exam trap
Test-takers frequently think increasing Kerberos ticket lifetimes or enabling NTLM fallback will solve the issue, but they overlook the strict time synchronization requirement that is fundamental to Kerberos protocol security.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Synchronize all client workstation clocks using a centralized NTP server
Kerberos authentication relies on synchronized clocks between clients and domain controllers, with a default maximum time skew tolerance of 5 minutes (RFC 4120). A client clock drift of up to 10 minutes exceeds this tolerance, causing intermittent authentication failures because Kerberos ticket requests are rejected as invalid or replay attacks. Synchronizing all client workstations to a centralized NTP server resolves the time skew and restores Kerberos authentication reliability.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Implement password complexity policies to reduce authentication errors
Why it's wrong here
Password complexity policies, while crucial for overall security by making passwords harder to guess or crack, do not address authentication errors stemming from Kerberos time skew. Kerberos authentication failures due to time differences between the client, server, and Key Distribution Center (KDC) are protocol-level issues unrelated to the strength or format of the user's password. These policies prevent brute-force attacks but do not resolve clock synchronization problems.
- ✗
Enable NTLM fallback authentication for the web applications
Why it's wrong here
Enabling NTLM fallback authentication might bypass Kerberos time skew issues because NTLM does not rely on strict time synchronization for ticket issuance. However, NTLM is an older, less secure authentication protocol compared to Kerberos, making it susceptible to various attacks like pass-the-hash. Using it as a permanent solution significantly degrades the enterprise's security posture and should be avoided in environments where Kerberos is the primary and preferred authentication mechanism.
- ✓
Synchronize all client workstation clocks using a centralized NTP server
Why this is correct
Kerberos relies heavily on precise time synchronization between the client, the authenticating server, and the Key Distribution Center (KDC) to prevent replay attacks and ensure ticket validity. By synchronizing all client workstation clocks with a centralized Network Time Protocol (NTP) server, the enterprise ensures that all systems operate within the acceptable time skew tolerance (typically 5 minutes) required by Kerberos. This direct approach eliminates the root cause of authentication failures related to clock differences, allowing Kerberos to function securely and efficiently.
- ✗
Configure Kerberos ticket lifetimes to 24 hours to reduce sensitivity to time skew
Why it's wrong here
Configuring Kerberos ticket lifetimes to 24 hours does not resolve time skew issues; it merely extends the period a valid ticket can be used, which is distinct from the initial authentication process's time synchronization requirement. While a longer lifetime might reduce the frequency of re-authentication, it does not address the underlying clock difference that causes initial ticket issuance failures. Furthermore, increasing ticket lifetimes can significantly expand the attack surface, as a compromised ticket remains valid for a longer duration, increasing the window for replay attacks or unauthorized access.
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Related to this question
Learn chapter
Security Governance and Principles
Key term
Kerberos
Kerberos is a network authentication protocol that uses tickets and symmetric-key cryptography to verify the identity of users and services in a secure, non-repudiable way.
Key term
Security
Security in IT is the practice of protecting systems, networks, and data from unauthorized access, damage, or theft.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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