The answer is to import the vendor's private CA certificate into the FortiGate's trusted root CA store. This resolves the ERR_CERT_AUTHORITY_INVALID error because during full SSL inspection, the FortiGate acts as a man-in-the-middle and must validate the entire certificate chain of the destination server; if the vendor’s private CA is not trusted by the FortiGate, it cannot complete the handshake, causing the browser to reject the connection. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of SSL inspection certificate error fix procedures and the distinction between client-side and FortiGate-side trust stores—a common trap is thinking that installing the CA on clients alone is sufficient. Remember the memory tip: “Gate first, then client” — the FortiGate must trust the upstream CA before it can re-sign the certificate for the user.
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.
A company with 500 users has a FortiGate 1000D running FortiOS 7.2. They have configured full SSL inspection and web filtering to block malware and phishing sites. The administrator receives complaints that some users cannot access a legitimate business website (https://vendor.example.com). The administrator checks the FortiGate logs and sees that the connection is allowed by the firewall policy and web filter. However, the user's browser shows 'ERR_CERT_AUTHORITY_INVALID'. The administrator verifies that the FortiGate's CA certificate is installed on all client machines. Further investigation reveals that the vendor's website uses a certificate signed by a private CA that is not trusted by the FortiGate. The administrator wants to resolve the issue without disabling SSL inspection for the whole website or compromising security. What should the administrator do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Import the vendor's private CA certificate into the FortiGate's trusted root CA store.
The FortiGate cannot validate the vendor's certificate because its private CA is not in the FortiGate's trusted root store. By importing that CA certificate into the FortiGate's trusted root CA store, the FortiGate will trust the vendor's certificate chain, allowing full SSL inspection to proceed without errors. This resolves the ERR_CERT_AUTHORITY_INVALID error while maintaining security inspection for the domain.
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.
✗
Create an SSL exemption for the vendor's domain in the SSL inspection profile.
Why it's wrong here
Exemption would bypass inspection, allowing traffic but reducing security.
✓
Import the vendor's private CA certificate into the FortiGate's trusted root CA store.
Why this is correct
This allows the FortiGate to validate the vendor's certificate and issue a trusted session certificate.
Related concept
Read the scenario before looking for a memorised answer.
✗
Change the SSL inspection profile to certificate inspection only.
Why it's wrong here
Certificate inspection would still fail because the FortiGate cannot validate the vendor's certificate.
✗
Install the vendor's CA certificate on the client machines.
Why it's wrong here
The client already trusts the FortiGate CA; the issue is the FortiGate not trusting the vendor's CA.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume the client-side CA certificate installation is sufficient, but the FortiGate itself must also trust the server's issuing CA to perform full SSL inspection without errors.
Detailed technical explanation
How to think about this question
During full SSL inspection, the FortiGate acts as a man-in-the-middle and must validate the server's certificate chain against its own trusted root store. If the server uses a private CA not in that store, the FortiGate cannot complete the handshake and returns an invalid certificate error to the client. Importing the private CA certificate into the FortiGate's trusted root store (under System > Certificates > CA Certificates) allows the FortiGate to trust the chain, enabling successful inspection without disabling security features.
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: Import the vendor's private CA certificate into the FortiGate's trusted root CA store. — The FortiGate cannot validate the vendor's certificate because its private CA is not in the FortiGate's trusted root store. By importing that CA certificate into the FortiGate's trusted root CA store, the FortiGate will trust the vendor's certificate chain, allowing full SSL inspection to proceed without errors. This resolves the ERR_CERT_AUTHORITY_INVALID error while maintaining security inspection for the domain.
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.
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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