The answer is that the firewall's CA certificate is not installed or trusted on the affected users' endpoints. This is the most likely cause because SSL Forward Proxy decryption works by intercepting the client’s request, then the firewall generates a new certificate on-the-fly for the destination server, signed by the firewall’s own CA certificate. If that CA certificate is not present in the local trusted root store on a user’s machine, the browser will flag the connection with a certificate warning, even though the destination server itself has a valid public certificate and no threats are detected. On the PCNSA exam, this scenario tests your understanding of how SSL Forward Proxy decryption relies on a trusted CA chain on endpoints, not just on the firewall policy; a common trap is assuming the issue is with the application’s certificate or network connectivity. Memory tip: “No trust, no decrypt—the endpoint must trust the proxy’s CA to avoid the warning.”
PCNSA Decryption and Monitoring Practice Question
This PCNSA practice question tests your understanding of decryption and monitoring. 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 mid-sized enterprise has deployed a Palo Alto Networks firewall with SSL Forward Proxy decryption for outbound traffic. The firewall uses a CA-signed certificate from a public CA, and the certificate is installed on all corporate-managed endpoints. Recently, the security team noticed that a few users are unable to access a specific external SaaS application (app.example.com) over HTTPS. Other users can access it without issues. The firewall logs show that for these users, the session is being decrypted and no threat is detected. The application uses a valid certificate from a public CA. The affected users are in the same IP subnet and use the same browser version. Which is the most likely cause?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall's CA certificate is not installed or trusted on the affected users' endpoints.
Option C is correct because SSL Forward Proxy decryption requires the firewall to generate a new certificate on-the-fly for the destination server, signed by the firewall's own CA certificate. If the CA certificate is not trusted on the affected users' endpoints, the browser will display a certificate warning or block the connection entirely, even though the decryption policy is applied and no threats are detected. Since other users in the same subnet can access the application, the issue is isolated to the trust store on the affected machines, not the network or decryption policy.
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 decryption policy is set to 'No Decrypt' for the affected users' source IP range.
Why it's wrong here
Logs show decryption is happening, so policy is not bypassing.
✗
The firewall is performing SSH proxy instead of SSL decryption for those users.
The firewall's CA certificate is not installed or trusted on the affected users' endpoints.
Why this is correct
Without trust, the browser rejects the decrypted connection.
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 SaaS application's certificate has expired for those users due to time zone differences.
Why it's wrong here
The certificate is valid globally; time zone differences do not affect certificate validity.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume the decryption policy is misconfigured or that the server certificate is invalid, but the key detail is that the firewall logs show decryption is occurring and no threats are detected, pointing to a client-side trust issue rather than a policy or server problem.
Trap categories for this question
Command / output trap
Logs show decryption is happening, so policy is not bypassing.
Detailed technical explanation
How to think about this question
In SSL Forward Proxy, the firewall intercepts the ClientHello, then initiates a new TLS handshake with the server, presenting the server's certificate to the firewall's internal engine. The firewall then generates a new certificate for the server's domain, signed by the firewall's CA certificate, and presents that to the client. If the client does not trust the firewall's CA certificate (e.g., it is not in the Trusted Root Certification Authorities store), the browser will reject the connection with a security error (e.g., NET::ERR_CERT_AUTHORITY_INVALID). This is a common issue when deploying decryption in phases or when endpoints are not centrally managed.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Decryption and Monitoring — This question tests Decryption and Monitoring — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall's CA certificate is not installed or trusted on the affected users' endpoints. — Option C is correct because SSL Forward Proxy decryption requires the firewall to generate a new certificate on-the-fly for the destination server, signed by the firewall's own CA certificate. If the CA certificate is not trusted on the affected users' endpoints, the browser will display a certificate warning or block the connection entirely, even though the decryption policy is applied and no threats are detected. Since other users in the same subnet can access the application, the issue is isolated to the trust store on the affected machines, not the network or decryption policy.
What should I do if I get this PCNSA 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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Question Discussion
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