This indicates an attack attempt against a policy bypass vulnerability in Fortinet's FortiWeb Web Application Firewall.
The vulnerability is due to an input validation error when handling overly large HTTP requests. A remote attacker can exploit this to gain unauthorized access to sensitive information.
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Fortinet recommends to enable "Block Malformed Request" violation in "Protocol Constraints". In current versions of FortiWeb, this may be found under the Web Protection -> Protocol form.
Fortinet is working to a flexible solution for v4.3 Patch 7 which will further address this issue.