This indicates an attack attempt against an Arbitrary File Upload vulnerability in ClipBucket.
The vulnerability is caused by insufficient sanitizing of user-uploaded files when handling an unauthenticated file upload. It allows a remote attacker to upload an arbitrary file onto vulnerable systems.
ClipBucket before 4.0.0 Release 4902
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: