This indicates an attack attempt against a Zero-Day vulnerability protected by the signature which has been released by Fortinet's FortiGuard Labs.
This signature should help mitigate the Zero-Day threat proactively both prior to, and after an official fix is available from the vendor. Once this official fix is available, further details will be made available in an advisory on our FortiGuard Center (http://www.fortiguard.com). This signature and description will also be updated at that point in time.
This is a Zero-Day (unpatched) vulnerability that is currently being investigated by Fortinet's FortiGuard Labs.
Any Zero-Day vulnerabilities can have a large impact due to the fact that they remain unpatched, making exploit attempts more effective since the given attack vector remains open.