With the limited information that is available, we can infer that this vulnerability affects the terminal console functionality within the Fortigate admin panel. This vulnerability allows an unauthenticated attacker to create administrative accounts on the Fortinet device.
ArcticWolf have observed a handful of exploitations of this vulnerability in early December, where an unauthenticated threat actor has created administrative accounts and changed device configurations. They have listed a number of IoC's which can help with identifying any malicious activity on devices. Fortinet have also released similar IoC's for this vulnerability.
Fortinet have released patching information and their own IoC's here.