The purpose of the SSL Forward Untrust certificate is to handle sessions where the original server certificate is invalid or untrusted (e.g., expired, self-signed, wrong hostname). To ensure the end-user is alerted to this potential security risk, the firewall must present a certificate that the user's browser will not trust. A self-signed certificate generated on the firewall is inherently untrusted by client browsers unless it has been explicitly installed in their trust stores. Using this self-signed certificate for the Forward Untrust role correctly triggers a browser security warning, propagating the original trust issue to the user as required.

A. A web server certificate signed by the organization's PKI: This would be trusted by internal clients, thus hiding the certificate warning from the user, which is contrary to the goal.

C. A subordinate Certificate Authority certificate signed by the organization's PKI: This is the best practice for the SSL Forward Trust certificate, not the Forward Untrust certificate. It is designed to be trusted.

D. A web server certificate signed by an external Certificate Authority: This would be trusted by all standard browsers, which would prevent the user from receiving the necessary warning about the untrusted site.

1. Palo Alto Networks Administrator's Guide - PAN-OS 10.2

"Configure SSL Forward Proxy": "For sites with untrusted certificates

the firewall uses the Forward Untrust Certificate to present to the client so that the user sees a block page. If you do not configure a Forward Untrust certificate

the firewall uses a self-signed certificate by default." This confirms that a self-signed certificate is the standard and default mechanism for this function.

2. Palo Alto Networks Administrator's Guide - PAN-OS 10.2

"Decrypt a Server Session with an Untrusted Certificate": "When the firewall detects an untrusted server certificate

it presents a copy of the Forward Untrust Certificate to the client to block the session and inform the user that the site is untrusted." This directly states the function is to inform the user of the untrusted status.

3. Palo Alto Networks Administrator's Guide - PAN-OS 10.2

"Certificates": This section describes a self-signed certificate as one that is signed by its own private key and not by a trusted CA. This inherent lack of external validation is why it serves the "untrust" function perfectly in this scenario.