Not all websites send their complete certificate
chain even though the
RFC 5246 TLSv1.2 standard requires
authenticated servers to provide a valid certificate chain leading
to an acceptable certificate authority. When you enable decryption
and apply a Forward Proxy Decryption profile that enables
Block
sessions with untrusted issuers
in the Decryption policy,
if an intermediate certificate is missing from the certificate list the
website’s server presents to the firewall, the firewall can’t construct
the certificate chain to the top (root) certificate. In these cases,
the firewall presents its Forward Untrust Certificate to the client
because the firewall cannot construct the chain to the root certificate
and trust cannot be established without the missing intermediate
certificate.