When a Vulnerability scanner is run in a network then it shows this vulnerability for firewall.
Untrusted TLS/SSL server X.509 certificate.
Description:
The server's TLS/SSL certificate is signed by a Certification Authority (CA) that is not well-known or trusted. This could happen if: the chain/intermediate certificate is missing, expired or has been revoked; the server hostname does not match that configured in the certificate; the time/date is incorrect; or a self-signed certificate is being used. The use of a self-signed certificate is not recommended since it could indicate that a TLS/SSL man-in-the-middle attack is taking place