The correct answer is that the ACL permits SSH from any source in 10.0.0.0/8 to the internal host, allowing an external attacker to spoof an internal IP. This security gap exists because the access control list does not filter based on the legitimacy of the source address; it simply trusts that any packet claiming to come from the private 10.0.0.0/8 range is internal. Since RFC 1918 addresses are non-routable on the public Internet, an attacker can perform ACL spoofing via private IP addresses by forging a source IP in that range, and the border router will forward the SSH traffic to 192.168.1.100 as if it originated from inside the network. On the CISSP exam, this scenario tests your understanding of anti-spoofing controls and the principle of implicit deny—a common trap is assuming private IP ranges are automatically blocked at the perimeter. Remember the memory tip: "Private IPs on public interfaces are a spoofing invitation; always add an inbound deny for RFC 1918 space."
CISSP Asset Security Practice Question
This CISSP practice question tests your understanding of asset security. 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
Access control list (ACL) extract from a Cisco router:
!
access-list 100 permit tcp 10.0.0.0 0.255.255.255 any eq 443
access-list 100 permit tcp 10.0.0.0 0.255.255.255 host 192.168.1.100 eq 22
access-list 100 deny tcp any host 10.0.0.1 eq 80
access-list 100 permit ip any any
!
interface GigabitEthernet0/0
ip access-group 100 in
!
Refer to the exhibit. An organization uses this ACL on the external interface of a border router to control access to internal services. A security analyst discovered that an attacker from the Internet was able to SSH into the internal server at 192.168.1.100. Which of the following is the MOST likely reason for this security gap?
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.
Access control list (ACL) extract from a Cisco router:
!
access-list 100 permit tcp 10.0.0.0 0.255.255.255 any eq 443
access-list 100 permit tcp 10.0.0.0 0.255.255.255 host 192.168.1.100 eq 22
access-list 100 deny tcp any host 10.0.0.1 eq 80
access-list 100 permit ip any any
!
interface GigabitEthernet0/0
ip access-group 100 in
!
A
The final permit ip any any rule allows all traffic, bypassing the other rules
Why wrong: The final rule permits all traffic not explicitly denied, but the SSH traffic is permitted earlier, so this is not the gap.
B
The ACL permits HTTP traffic to host 10.0.0.1 which could be used as a pivot
Why wrong: The HTTP rule denies traffic to 10.0.0.1 port 80, so it is not the cause.
C
The ACL permits SSH from any source in 10.0.0.0/8 to the internal host, allowing an external attacker to spoof an internal IP
An attacker can spoof an IP address in the 10.0.0.0/8 range to bypass the ACL and SSH into the server.
D
The ACL is applied inbound on the external interface, so it should block all external traffic, but the permit rules override
Why wrong: Inbound on the external interface, the ACL examines traffic coming into the interface from outside, so the permit rules apply to traffic originating from outside, which is the intended filter point.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL permits SSH from any source in 10.0.0.0/8 to the internal host, allowing an external attacker to spoof an internal IP
Option C is correct because the ACL permits SSH (TCP port 22) from any source within the 10.0.0.0/8 range to the internal host 192.168.1.100. Since 10.0.0.0/8 is a private RFC 1918 address space, an external attacker can spoof a source IP in that range, bypassing the intended restriction that only internal hosts should use those addresses. The ACL does not validate the source IP's legitimacy, so a spoofed packet from the Internet matching the permit rule will be allowed through.
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 final permit ip any any rule allows all traffic, bypassing the other rules
Why it's wrong here
The final rule permits all traffic not explicitly denied, but the SSH traffic is permitted earlier, so this is not the gap.
✗
The ACL permits HTTP traffic to host 10.0.0.1 which could be used as a pivot
Why it's wrong here
The HTTP rule denies traffic to 10.0.0.1 port 80, so it is not the cause.
✓
The ACL permits SSH from any source in 10.0.0.0/8 to the internal host, allowing an external attacker to spoof an internal IP
Why this is correct
An attacker can spoof an IP address in the 10.0.0.0/8 range to bypass the ACL and SSH into the server.
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 ACL is applied inbound on the external interface, so it should block all external traffic, but the permit rules override
Why it's wrong here
Inbound on the external interface, the ACL examines traffic coming into the interface from outside, so the permit rules apply to traffic originating from outside, which is the intended filter point.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the misconception that an ACL's permit rule for a private IP range is safe because those addresses are not routable on the Internet, but the trap is that attackers can spoof those source IPs, and the ACL will allow the traffic if it matches the permit criteria.
Detailed technical explanation
How to think about this question
In Cisco ACLs, source IP spoofing is a well-known bypass technique because ACLs operate at Layer 3 and do not perform reverse path forwarding (RPF) checks unless uRPF is explicitly configured. The 10.0.0.0/8 block is non-routable on the public Internet, but a router cannot distinguish a spoofed private source IP from a legitimate one without additional anti-spoofing measures like ingress filtering (BCP 38). In real-world deployments, such ACLs should include explicit deny statements for RFC 1918 addresses on external interfaces to prevent spoofing.
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 analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Asset Security — This question tests Asset Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL permits SSH from any source in 10.0.0.0/8 to the internal host, allowing an external attacker to spoof an internal IP — Option C is correct because the ACL permits SSH (TCP port 22) from any source within the 10.0.0.0/8 range to the internal host 192.168.1.100. Since 10.0.0.0/8 is a private RFC 1918 address space, an external attacker can spoof a source IP in that range, bypassing the intended restriction that only internal hosts should use those addresses. The ACL does not validate the source IP's legitimacy, so a spoofed packet from the Internet matching the permit rule will be allowed through.
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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