The vulnerability stemmed from insufficient validation when setting object state during deserialization. The commit diff shows critical additions:- 1) check_state_key() to block forbidden keys- 2) Modified set_python_instance_state() to enforce the blacklist. The unpatched FullLoader's set_python_instance_state allowed attackers to set arbitrary attributes (like reduce) via crafted YAML, leading to code execution. The construct_python_object_apply method triggered this unsafe state-setting process. These functions were directly modified in the security patch to add validation.