This indicates an attack attempt to exploit a Heap Buffer Overflow vulnerability in Nagios.
The vulnerability is due to an improper boundary check condition in the application when handling an excessively long host parameter value. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
Nagios Enterprises Nagios 3.4.3 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are unaware of any vendor supplied patch or updates available for this issue.