The SSL-VPN feature of FortiOS 4.3.12 and lower only checks the first byte of the TLS MAC in the finished message.
An attacker may intercept encrypted packets in transit and modifying their contents by changing the middle or the end of the MAC field in the TLS finished message.
A remote attacker may be able to modify the contents of an encrypted TLS packet without detection of the modifications when the SSL-VPN feature is configured.Fortinet is not aware of any exploit in the wild.
FortiOS 4.3.12 and lower.
Customers using the SSL-VPN feature and running FortiOS<= 4.3.12 must upgrade to FortiOS 4.3.13 / 5.0.x / 5.2.x.
Thanks to Yngve N. Pettersen for working with us to help protect customers.