What is it?
Fortinet offers virtual private network (SSL VPN) and firewall products as part of FortiOS and FortiProxy.
Why is it a risk?
An attacker can overflow a buffer to allow remote, unauthorized, execution of arbitrary code.
This is considered a CRITICAL problem of VERY HIGH risk.
How can you mitigate the risk?
The only mitigation is to upgrade to the latest version of FortiOS or FortiProxy (link below).
Resources:
FortiNet