This indicates an attack attempt to exploit a Code Execution Vulnerability in Tenable Appliance.
The vulnerability is due to improper validation of user supplied inputs when handling a crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary code on affected machines.
Tenable Appliance 3.5 - 4.4.0
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the latest update from the vendor