The answer is the OUTPUT chain default policy being set to ACCEPT, which creates a critical security issue by permitting any outbound traffic without restriction. This is technically dangerous because a default ACCEPT policy on the OUTPUT chain means that if the server is compromised—especially one directly connected to the internet—an attacker can freely exfiltrate sensitive data, establish outbound connections to command-and-control servers, or launch further attacks without any firewall-level barrier. On the CompTIA SecurityX CAS-004 exam, this concept tests your understanding of defense in depth and the principle of least privilege applied to firewall rules; a common trap is focusing only on inbound rules while ignoring that outbound controls are equally vital for preventing data exfiltration. To remember this, think of the mnemonic “OUTbound = OUTgoing threats”—a default DROP on OUTPUT forces explicit permission for every outbound connection, closing the door on covert channels.
CAS-004 Security Engineering Practice Question
This CAS-004 practice question tests your understanding of security engineering. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An administrator runs the above iptables command on a Linux server. The server is directly connected to the internet. Which of the following is the MOST significant security issue with this configuration?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The OUTPUT chain default policy is ACCEPT, allowing any outbound traffic
The most significant security issue is that the OUTPUT chain default policy is ACCEPT, allowing any outbound traffic from the server. Since the server is directly connected to the internet, this means that if the server is compromised, an attacker can freely exfiltrate data, initiate outbound connections to command-and-control servers, or perform other malicious activities without any firewall restriction. A default DROP policy on the OUTPUT chain would require explicit rules for allowed outbound traffic, providing defense in depth.
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 INPUT chain default policy is DROP, which will block legitimate traffic
Why it's wrong here
DROP is a secure default.
✓
The OUTPUT chain default policy is ACCEPT, allowing any outbound traffic
Why this is correct
This could allow data exfiltration or command and control traffic.
Related concept
Read the scenario before looking for a memorised answer.
✗
SSH (port 22) is allowed from any source, which is overly permissive
Why it's wrong here
SSH may be needed, but the rule is permissive; however, the output policy is the bigger issue.
✗
ICMP echo requests are allowed, which could be used for network reconnaissance
Why it's wrong here
While true, it's less critical than the outbound issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates focus on inbound rules (like SSH or ICMP) because they are more familiar, but the exam tests understanding that a default ACCEPT on the OUTPUT chain is a critical security gap that enables data exfiltration and is often overlooked in firewall configurations.
Trap categories for this question
Command / output trap
SSH may be needed, but the rule is permissive; however, the output policy is the bigger issue.
Detailed technical explanation
How to think about this question
In iptables, the OUTPUT chain controls packets generated by the local system. A default ACCEPT policy means no filtering is applied to outbound connections, which violates the principle of least privilege. In a real-world scenario, a compromised web server with a default ACCEPT OUTPUT could be used to exfiltrate a database of customer records via an outbound HTTPS connection to an attacker's server, bypassing any egress monitoring if not separately inspected. The RFC 2979 (Behavior of and Requirements for Internet Firewalls) recommends that firewalls should filter both inbound and outbound traffic to enforce security policies.
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 practitioner preparing for the CAS-004 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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.
Security Engineering — This question tests Security Engineering — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The OUTPUT chain default policy is ACCEPT, allowing any outbound traffic — The most significant security issue is that the OUTPUT chain default policy is ACCEPT, allowing any outbound traffic from the server. Since the server is directly connected to the internet, this means that if the server is compromised, an attacker can freely exfiltrate data, initiate outbound connections to command-and-control servers, or perform other malicious activities without any firewall restriction. A default DROP policy on the OUTPUT chain would require explicit rules for allowed outbound traffic, providing defense in depth.
What should I do if I get this CAS-004 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
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network administrator is troubleshooting connectivity issues. Based on the exhibit, which of the following is true about the iptables rules?
hard
A.New connections from the internet to 10.0.1.0/24 are allowed.
B.All traffic is allowed by default because the policy is ACCEPT.
✓ C.Traffic from 10.0.1.0/24 to the internet is allowed, but return traffic is only allowed if it is part of an established connection.
D.The rules apply to incoming traffic on the firewall itself.
Why C: Option C is correct because the iptables rules show a default FORWARD policy of ACCEPT, but the INPUT chain has a default policy of DROP. The rule '-A FORWARD -s 10.0.1.0/24 -j ACCEPT' allows outbound traffic from the 10.0.1.0/24 subnet to the internet. The rule '-A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT' ensures that only return traffic belonging to an existing connection is permitted back, while new inbound connections from the internet are implicitly dropped by the FORWARD chain's default policy.
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Question Discussion
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