There seems to be a vulnerarbility in some FortiMail versions, that allow an unauthenticated remote attacker to access the system by requesting a password change. Please refer to the FortiGuard PSIRT article.
The problem here is not only the unauthorized access to the system, but also the change of the password of all configured administrative accounts. Also, the maintainer functionality to reset the administrator password over a serial console of the FortiMail is being disabled from the attacker.
FortiOS v6.2 has been released in March this year and we are still gaining experience with this version. In this article we would like to draw you attention to the protocol which is used for FortiGuard service communication. Up to v6.0 udp has been used, with 6.2 the default protocol has changed to https.