This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Synology DiskStation Manager.
The vulnerability is due to a design flaw in the way the vulnerable software handles user controlled data. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
Synology DiskStation Manager 4.0-2257
Synology DiskStation Manager 4.1-2851
Synology DiskStation Manager 4.2-3236
Synology DiskStation Manager 4.3-3810
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.