The issue arises because the firewall does not trust the certificate authority (CA) that signed the certificate for https://www.very-important-website.com. Consequently, during SSL Forward Proxy, the firewall uses its configured "Forward Untrust Certificate" (Untrusted-CA) to re-sign the certificate, which the end-user's browser does not trust.

To resolve this, the firewall's trust store must be updated to recognize the website's CA (Well-Known-Root-CA) as trusted. By importing the root and intermediate CAs into the firewall and marking the root as a "Trusted Root CA," the firewall will correctly validate the website's certificate. It will then use its "Forward Trust Certificate" for re-signing, which clients are expected to trust, thus eliminating the warning for this specific site while still flagging genuinely untrusted sites.

A: The end-user systems already trust the Well-Known-Root-CA chain, as stated in the question. The problem is their lack of trust in the firewall's re-signing certificate (Untrusted-CA).

B: Clearing the "Forward Untrust Certificate" option in the decryption profile would cause the firewall to block, not just warn, users trying to access sites with untrusted certificates, failing to meet the requirement for the important website.

D: Device Certificates is the incorrect location. This area is for certificates that identify the firewall itself, not for the certificate authority trust store used to validate external servers.

1. Palo Alto Networks Documentation - Configure SSL Forward Proxy: "For the firewall to be able to decrypt and inspect the SSL/TLS traffic

you must set up a Forward Trust certificate and a Forward Untrust certificate... If the server certificate is signed by a CA that the firewall trusts

the firewall re-signs the server certificate with the Forward Trust certificate... If the server certificate is signed by a CA that the firewall does not trust

the firewall re-signs it with the Forward Untrust certificate." This explains the mechanism causing the issue.

2. Palo Alto Networks Documentation - Certificate Management: "The firewall uses certificates to secure communication and authenticate entities... To enable the firewall to validate the certificate chains of the servers it communicates with

you must load the CA certificates that sign those server certificates onto the firewall and configure them as trusted CAs." This directly supports the solution in option C. The section on "Import a Certificate" details the process of adding a CA to the trust store.

3. Palo Alto Networks Documentation - Decryption Concepts: "The firewall maintains its own list of trusted CAs... If the CA that signed the server certificate is not on the firewall’s trusted CA list

the firewall considers the server certificate untrusted." This confirms that the root cause is the firewall's trust store

which option C correctly addresses.