The answer is that the ACL’s failure to filter outbound traffic is the most significant governance concern because it allows spoofed internal IP addresses to exit the network, directly violating anti-spoofing best practices such as RFC 2827 and BCP 38. This omission means the border router only inspects inbound traffic on its external interface, leaving the network exposed to IP spoofing attacks that can originate from compromised internal hosts or misconfigured devices. On the Certified Information Security Manager CISM exam, this scenario tests your understanding of perimeter defense governance, specifically how ACLs must enforce bidirectional anti-spoofing controls to align with security policy frameworks. A common trap is focusing only on inbound rules, but governance requires verifying that outbound traffic also prevents source address spoofing. Memory tip: think “inbound protects you, outbound protects others”—a border router ACL must guard both directions to meet governance standards.
CISM Information Security Governance Practice Question
This CISM practice question tests your understanding of information security governance. 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.
Access Control List (ACL) on border router:
access-list 100 deny ip 10.0.0.0 0.255.255.255 any
access-list 100 deny ip 172.16.0.0 0.15.255.255 any
access-list 100 deny ip 192.168.0.0 0.0.255.255 any
access-list 100 deny ip 127.0.0.0 0.255.255.255 any
access-list 100 deny ip 0.0.0.0 0.255.255.255 any
access-list 100 permit ip any any
Refer to the exhibit. A security analyst reviews the ACL on the organization's border router. Based on the exhibit, which of the following is the MOST significant governance concern?
Refer to the exhibit.
Access Control List (ACL) on border router:
access-list 100 deny ip 10.0.0.0 0.255.255.255 any
access-list 100 deny ip 172.16.0.0 0.15.255.255 any
access-list 100 deny ip 192.168.0.0 0.0.255.255 any
access-list 100 deny ip 127.0.0.0 0.255.255.255 any
access-list 100 deny ip 0.0.0.0 0.255.255.255 any
access-list 100 permit ip any any
A
The ACL is applied to the outbound interface, which is ineffective for blocking inbound attacks.
Why wrong: The exhibit does not specify direction, but the ACL is likely applied inbound; even if outbound, the governance concern is the missing outbound filtering.
B
The ACL does not include filtering for outbound traffic, which may allow spoofed internal IPs to exit the network.
Outbound filtering (ingress filtering) is missing, which is a governance oversight.
C
The ACL permits any traffic after denying specific IP ranges, creating a security gap.
Why wrong: The permit any any at the end is standard after denying specific ranges; it does not create a gap by itself.
D
The ACL permits all traffic from private IP addresses, which could allow internal IP spoofing.
Why wrong: The ACL denies private IP addresses inbound, so this is not a concern.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL does not include filtering for outbound traffic, which may allow spoofed internal IPs to exit the network.
Option B is correct because the ACL shown only filters inbound traffic on the border router's external interface. Without an outbound ACL (or an inbound ACL on the internal interface), spoofed packets with internal source IP addresses can exit the network, enabling IP spoofing attacks that bypass anti-spoofing best practices (RFC 2827, BCP 38). This is a governance concern as it violates the principle of preventing source address spoofing, which is a fundamental security control for network perimeter defense.
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 ACL is applied to the outbound interface, which is ineffective for blocking inbound attacks.
Why it's wrong here
The exhibit does not specify direction, but the ACL is likely applied inbound; even if outbound, the governance concern is the missing outbound filtering.
✓
The ACL does not include filtering for outbound traffic, which may allow spoofed internal IPs to exit the network.
Why this is correct
Outbound filtering (ingress filtering) is missing, which is a governance oversight.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ACL permits any traffic after denying specific IP ranges, creating a security gap.
Why it's wrong here
The permit any any at the end is standard after denying specific ranges; it does not create a gap by itself.
✗
The ACL permits all traffic from private IP addresses, which could allow internal IP spoofing.
Why it's wrong here
The ACL denies private IP addresses inbound, so this is not a concern.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates focus on the inbound ACL's content (denying private IPs) and miss the governance issue of missing outbound anti-spoofing controls, which is a classic CISM governance concern about policy compliance rather than just ACL syntax.
Detailed technical explanation
How to think about this question
Under the hood, ACLs are processed top-down with an implicit deny at the end; the 'permit any' in the exhibit is the last explicit entry, meaning all traffic not matching earlier deny statements is permitted. In a real-world scenario, without an outbound ACL (or an inbound ACL on the internal interface), an attacker who compromises an internal host can spoof internal IPs to launch attacks against external targets, making the organization complicit in DDoS amplification or other attacks. RFC 2827 (BCP 38) mandates ingress filtering at the network edge to drop packets with source addresses that do not belong to the network, which is missing here.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Information Security Governance — This question tests Information Security Governance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL does not include filtering for outbound traffic, which may allow spoofed internal IPs to exit the network. — Option B is correct because the ACL shown only filters inbound traffic on the border router's external interface. Without an outbound ACL (or an inbound ACL on the internal interface), spoofed packets with internal source IP addresses can exit the network, enabling IP spoofing attacks that bypass anti-spoofing best practices (RFC 2827, BCP 38). This is a governance concern as it violates the principle of preventing source address spoofing, which is a fundamental security control for network perimeter defense.
What should I do if I get this CISM question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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