This vulnerability affects the Fortinet FortiClient program. FortiClient is a client program used to connect to SSL/IPsec VPN endpoints.
A setting, disabled by default, enables FortiClient on the logon screen to allow users to connect to a VPN profile before logon. An attacker, with physical, or remote (e.g. through TSE, VNC…), access to a machine with FortiClient and this feature enabled, can obtain SYSTEM level privileges from the lock screen. No account or prior knowledge is required.
The vulnerability lies in the confirmation dialog shown when the server certificate is not valid (e.g. default auto-signed certificate, or Man-In-The-Middle with SSL/TLS interception situation).