Locate and install missing intermediate certificates to fix incomplete certificate
chains using the Decryption log.
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 (CA). When you
enable decryption and apply a Forward Proxy Decryption profile that
blocks sessions with untrusted issuers to a Decryption
policy rule, 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 trust can't be
established without the missing intermediate certificate.
The firewall also presents its Forward Untrust certificate
if traffic matches a Decryption profile that allows sessions with untrusted
issuers.
If a website you need to communicate
with for business purposes has one or more missing intermediate certificates and the
Decryption profile blocks sessions with untrusted issuers, then you can find and
download the missing intermediate certificate and install it on the firewall as a
trusted root CA so that the firewall trusts the site’s server. (The alternative is
to contact the website owner and ask them to configure their server so that it sends
the intermediate certificate during the handshake.)
If you allow
sessions with untrusted issuers in the Decryption profile, the firewall can
establish sessions even if the issuer is untrusted. The firewall presents the
Forward Untrust certificate to the client and displays a warning message in the
browser, which enables users to accept the risk and continue to the site or not.
However, it's a best practice to block sessions with untrusted issuers for
better security.