The application relates to controlling access in a software-defined network (SDN). A controller in the SDN receives an access request from an application program. The controller determines whether an operation on a resource as specified in the access request belongs to a permission list corresponding to the application program. The permission list includes a list of permitted operations on the resource by the application program. When the operation as specified in the access request belongs to the permission list, the controller sends a reply message allowing access by the application program. In this way, accesses by the application program are restricted according to the permission list, and malicious attacks from the application program can be prevented to ensure network security.