The answer is Rule 30, the implicit deny all entry at the end of a default Cisco IOS ACL, which poses the greatest risk during a firewall rule risk assessment. This rule is dangerous because it silently blocks all traffic that is not explicitly permitted, and if an administrator fails to add a preceding permit statement for critical services, legitimate traffic is denied without any alert, causing unexpected outages or security gaps. On the CISA exam, this tests your understanding of access control logic and the hidden dangers of implicit deny—a common trap is assuming that a default deny rule is always safe, when in fact it can be the root cause of a major operational risk if not properly managed. Remember the memory tip: “The last rule is the first to break you”—always verify that your permit statements come before the implicit deny to avoid a silent network failure.
CISA Practice Question: Information Systems Acquisition, Development and Implementation
This CISA practice question tests your understanding of information systems acquisition, development and implementation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
[admin@fw1]# show rules
Rule 10: allow from 10.0.1.0/24 to 10.0.2.0/24 dst-port 3306
Rule 20: allow from 10.0.1.0/24 to any dst-port 443
Rule 30: allow from any to 10.0.2.0/24 dst-port 22
During a security audit, which rule poses the greatest risk?
Refer to the exhibit.
[admin@fw1]# show rules
Rule 10: allow from 10.0.1.0/24 to 10.0.2.0/24 dst-port 3306
Rule 20: allow from 10.0.1.0/24 to any dst-port 443
Rule 30: allow from any to 10.0.2.0/24 dst-port 22
A
Rule 20
Why wrong: Rule 20 allows HTTPS from a specific source to any destination, which is low risk.
B
Rule 30
Rule 30 allows SSH from any source, posing a high risk.
C
None of the rules pose a risk
Why wrong: Rule 30 is clearly a risk.
D
Rule 10
Why wrong: Rule 10 restricts access to a specific source and port (MySQL).
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Rule 30
Rule 30 is the correct answer because it is a default Cisco IOS access control list (ACL) entry that implicitly denies all IP traffic. In a security audit, an implicit deny rule at the end of an ACL poses the greatest risk if it is not explicitly configured, as it can block legitimate traffic without the administrator's awareness, leading to unintended network outages or security gaps. This risk is highest when the ACL is applied to a critical interface without a preceding permit statement for required services.
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.
✗
Rule 20
Why it's wrong here
Rule 20 allows HTTPS from a specific source to any destination, which is low risk.
✓
Rule 30
Why this is correct
Rule 30 allows SSH from any source, posing a high risk.
Related concept
Read the scenario before looking for a memorised answer.
✗
None of the rules pose a risk
Why it's wrong here
Rule 30 is clearly a risk.
✗
Rule 10
Why it's wrong here
Rule 10 restricts access to a specific source and port (MySQL).
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on the explicit rules (Rule 10 or Rule 20) and overlook the hidden implicit deny rule, which is the most dangerous because it can silently block all traffic if not accounted for.
Detailed technical explanation
How to think about this question
In Cisco IOS, every ACL ends with an implicit deny any statement, which is not displayed in the running configuration but is enforced by the router. This rule matches all packets not previously permitted or denied, and it drops them silently without logging unless an explicit 'deny any log' is added. A real-world scenario is when an administrator applies an ACL to a VTY line for SSH access but forgets to include a permit statement for the management subnet; the implicit deny then blocks all remote access, locking the administrator out of the device.
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 Systems Acquisition, Development and Implementation — This question tests Information Systems Acquisition, Development and Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Rule 30 — Rule 30 is the correct answer because it is a default Cisco IOS access control list (ACL) entry that implicitly denies all IP traffic. In a security audit, an implicit deny rule at the end of an ACL poses the greatest risk if it is not explicitly configured, as it can block legitimate traffic without the administrator's awareness, leading to unintended network outages or security gaps. This risk is highest when the ACL is applied to a critical interface without a preceding permit statement for required services.
What should I do if I get this CISA 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This CISA practice question is part of Courseiva's free ISACA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CISA exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.