The answer is to use a bastion host to terminate TLS on behalf of the legacy application and forward via one-way replication. This is correct because the bastion host acts as a secure intermediary, wrapping the legacy application’s clear-text traffic in TLS encryption before it enters the cardholder data environment (CDE), while one-way replication ensures data flows only from the non-CDE segment to the CDE, preventing any reverse access that could compromise sensitive systems. On the CompTIA SecurityX CAS-004 exam, this scenario tests your understanding of PCI DSS compliance requirements for legacy application encryption, specifically how to isolate a CDE without modifying outdated software. A common trap is assuming a VPN or direct firewall rule is sufficient, but those still expose the CDE to unencrypted traffic or bidirectional risk. Memory tip: think “bastion as a bouncer”—it checks and wraps the traffic at the door, then lets only one-way data pass through.
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.
A financial institution is required to comply with PCI DSS and uses a mix of legacy and modern applications. The security architect proposes to segment the network so that the cardholder data environment (CDE) is isolated. However, a legacy application in a non-CDE segment must send data to a database in the CDE. The legacy application cannot be modified and communicates via clear-text protocols. Which of the following is the most secure solution that maintains compliance?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a bastion host to terminate TLS on behalf of the legacy application and forward via a one-way replication
A bastion host with TLS termination can wrap clear-text traffic in encryption, and strict firewall rules prevent direct access.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Replace the legacy application immediately
Why it's wrong here
Not feasible in the short term; doesn't solve current need.
✓
Use a bastion host to terminate TLS on behalf of the legacy application and forward via a one-way replication
Why this is correct
Encrypts traffic and limits the legacy application's direct access.
Related concept
Static NAT maps one inside address to one outside address.
✗
Place the legacy application in the CDE and isolate it with a firewall
Why it's wrong here
Expands the CDE scope and compliance burden.
✗
Install a network-based DLP sensor to monitor traffic
Why it's wrong here
Does not encrypt or isolate; compliance requires encryption.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CAS-004 NAT questions on configuration and troubleshooting.
Security Engineering — This question tests Security Engineering — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Use a bastion host to terminate TLS on behalf of the legacy application and forward via a one-way replication — A bastion host with TLS termination can wrap clear-text traffic in encryption, and strict firewall rules prevent direct access.
What should I do if I get this CAS-004 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CAS-004 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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