The vulnerability stems from insecure usage of BashOperator in the example_trigger_target_dag.py. The original implementation constructed bash_command by directly embedding dag_run.conf['message'] (user-controlled input) into the command string via Jinja templating. This allowed attackers to escape the command context and execute arbitrary commands. The patch moved user input to the 'env' parameter and used environment variable substitution, demonstrating the vulnerable pattern was in the BashOperator's bash_command parameter handling within this specific example DAG.