FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 192205
The Rapid7 vulnerability scanner reports the following security risk: "TLS/SSL Server Is Using Commonly Used Prime Numbers"

The detailed description is:
"The server is using a common or default prime number as a parameter during the Diffie-Hellman key exchange. This makes the secure session vulnerable to a precomputation attack. An attacker can spend a significant amount of time
to generate a lookup/rainbow table for a particular prime number. This lookup table can then be used to obtain the shared secret for the handshake and decrypt the session."

Standalone popular script/scanner ( also produces a similar report: "experimental Common prime with 2048 bits detected"

That risk report is triggered against FortiOS admin webUI port (usually port 443).

All FortiGate versions.

The feasibility of such an attack under a 2048-bit DH group is currently
assessed as uncertain and un-proven. FortiOS has no plan to address this
issue reported on the admin webUI port.

Customers should be advised to use a DH group of 2048 bits or above, by
using the following CLI command in FortiOS:

# conf system global
# set dh-params 2048
# end