This indicates an attack attempt to exploit a Heap Buffer Overflow Vulnerability in H2O Web Server.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
H2O Web Server prior to 2.2.5
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: