This indicates an attack attempt against a Buffer Overflow vulnerability in Fortinet's FortiGate.
The vulnerability is caused by an error when the vulnerable software handles a request with a malicious Cookie value. It allows a remote attacker to execute arbitrary code via a crafted HTTP query.
FortiGate 4.3.8 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the latest version of FortiOS.