IPSEC

If genuine VPN peers come from a known range of IPs, you can alleviate this with a local-in policy for UDP ports 500 and 4500. Set it up to allow known-good IPs (individual, ranges, subnets, GeoIP countries) and block everything else.

Or vice-versa, block known unexpected IPs.

Note that FortiOS has some basic DoS protection using IKE cookies in IKEv2. If number of connection attempts in SA_INIT stage reaches a certain number, FortiGate starts asking the peer to re-sent its SA_INIT with a provided cookie.

config system ike

set ike-embryonic-limit <number>

end

When the number of initiated SA_INITs is over half of this number, a cookie is required. If it reaches the number, FortiGate stops processing any new SA_INITs.

ref: https://docs.fortinet.com/document/fortigate/7.0.0/new-features/66410/ipsec-global-ike-embryonic-limit

Are you sure the high CPU is from this?  As @pminarik mentioned you can use a local-in policy for front-end with another firewall or your ISP's DDoS prevention.

Hello @psniech 

You can limit Ipsec access to a trusted host https://community.fortinet.com/t5/FortiGate/Technical-Tip-Restrict-IPSec-VPN-access-to-certain-countries/ta-p/192328

Thanks,

Pavan