ESXi and vCenter Server components communicate securely over SSL to ensure confidentiality, data integrity and authentication. Data is private, protected, and cannot be modified in transit without detection.
By default, vSphere services use the certificates that are created as part of the installation process and stored on each system. These default certificates are unique and make it possible to begin using the software, but they are not signed by a trusted certificate authority (CA).
To receive the full benefit of certificate checking, particularly if you intend to use SSL connections over the Internet, install new certificates that are signed by a valid internal certificate authority or purchase a certificate from a trusted security authority.