This indicates an attack attempt against a Remote Code Execution vulnerability in Dasan GPON.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
ZNID-GPON-25xx and certain H640-series ONTs
Device Compromise: Remote attackers can gain control of vulnerable devices.
Currently no official patch, you can patch it here