The Mobile VPN with SSL client makes an HTTPS connection to a Firebox. By default the web server on the Firebox has a self-signed certificate. This can cause a security alert to appear on the client computer when the Mobile VPN with SSL client connects. The security alert says that the Certificate Issuer for the site is untrusted or unknown.
The security alert appears because the default web server certificate for the Firebox is self-signed, and the certificate details do not match the external IP address of your Firebox.
It is safe to instruct users to click Yes on the security alert to proceed with the connection. After a user connects for the first time, the security alert does not appear for additional connections.
If you do not want users to see this security alert, you can configure a valid certificate for the web server on your Firebox. For more information, see Configure the Web Server Certificate for Firebox Authentication.