The answer is a cipher mismatch between the web server and the FortiGate’s re-encryption capabilities. When deep-inspection is active, the FortiGate decrypts the client’s traffic, inspects the content, and then must re-encrypt the data before sending it to the client. If the original web server uses a cipher suite that the FortiGate does not support for re-encryption—such as an obsolete or non-standard cipher—the FortiGate cannot complete the SSL handshake with the client, resulting in certificate errors or connection failures. On the Fortinet NSE 4 exam, this scenario tests your understanding of the SSL inspection flow and the critical distinction between decryption and re-encryption. A common trap is to blame the CA certificate or firewall policy, but since those are confirmed correct, the cipher mismatch is the root cause. Memory tip: think “Decrypt, inspect, re-encrypt—if the cipher doesn’t match, the handshake will catch.”
NSE4 Security Profiles Practice Question
This NSE4 practice question tests your understanding of security profiles. 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 configured SSL inspection with 'deep-inspection' profile. Users report that some websites fail to load with certificate errors. The firewall policy is correct. What is the most likely reason?
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 web server uses a cipher that the FortiGate cannot re-encrypt.
When deep-inspection is used, the FortiGate decrypts the client-to-server traffic, inspects the content, and then re-encrypts it before forwarding to the client. If the web server uses a cipher suite that the FortiGate does not support for re-encryption (e.g., an obsolete or non-standard cipher), the FortiGate cannot complete the SSL handshake with the client, causing certificate errors or connection failures. This is the most likely reason because the firewall policy is correct and the CA certificate is valid.
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 CA certificate has expired.
Why it's wrong here
An expired CA certificate would cause all SSL sites to fail.
✓
The web server uses a cipher that the FortiGate cannot re-encrypt.
Why this is correct
Some ciphers may not be supported for re-encryption, causing errors.
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 user's browser is outdated.
Why it's wrong here
Outdated browser may cause issues but not specifically certificate errors.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume certificate errors are always due to an expired CA certificate, but the question specifies that only some websites fail, which points to a cipher mismatch during re-encryption rather than a global CA issue.
Detailed technical explanation
How to think about this question
Under the hood, the FortiGate's deep-inspection proxy terminates the original SSL/TLS connection from the server and initiates a new SSL/TLS connection to the client using a dynamically generated certificate signed by the FortiGate's CA. The re-encryption cipher is chosen from the FortiGate's supported cipher list, which may not include weak or deprecated ciphers (e.g., RC4, 3DES, or export-grade ciphers) that the server originally used. In real-world scenarios, legacy servers or embedded devices often use such weak ciphers, causing the FortiGate to fail re-encryption and present a certificate error to the user.
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 NSE4 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 Profiles — This question tests Security Profiles — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The web server uses a cipher that the FortiGate cannot re-encrypt. — When deep-inspection is used, the FortiGate decrypts the client-to-server traffic, inspects the content, and then re-encrypts it before forwarding to the client. If the web server uses a cipher suite that the FortiGate does not support for re-encryption (e.g., an obsolete or non-standard cipher), the FortiGate cannot complete the SSL handshake with the client, causing certificate errors or connection failures. This is the most likely reason because the firewall policy is correct and the CA certificate is valid.
What should I do if I get this NSE4 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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