Answer:
The feature exploited was Cross Origin Resource Sharing (CORS)
Step-by-step explanation:
CORS is an HTML5 mechanism that allows you to share resources even outside of your domain boundaries and it simplifies how is done. CORS establishes several headers to help the system decide which requesters can be trusted and which are not. If security measures, such as properly configuring the Access-Control-Allow-Origin feature (that restricts and defines which domains are authorized to access your resources) are not implemented, then this can be used as a breach to steal private information from your domain.