Abstract—Software-Defined Networking (SDN) greatly meets the need in industry for programmable, agile, and dynamic networks by deploying diversified SDN applications on a central- ized controller. However, SDN application ecosystem inevitably introduces new security threats since compromised or malicious applications can significantly disrupt network operations. Thus, a number of effective security enhancement systems have been developed to defend against potential attacks from SDN appli- cations. In this paper, we identify a new vulnerability on flow rule installation in SDN, namely, buffered packet hijacking, which can be exploited by malicious applications to launch effective attacks bypassing all existing defense systems. The root cause of this vulnerability lies in that SDN systems do not check the inconsistency between buffer IDs and match fields when an application attempts to install flow rules. Thus, a malicious application can manipulate buffer IDs to hijack buffered packets even though they do not match any installed flow rules. We design effective attacks exploiting this vulnerability to disrupt all three SDN layers, i.e., application layer, data plane layer, and control layer. First, by modifying buffered packets and resending them to controllers, a malicious application can poison other applications. Second, by manipulating forwarding behaviors of buffered packets, a malicious application can not only disrupt TCP connections of flows but also make flows bypass network security policies. Third, by copying massive buffered packets to controllers, a malicious application can saturate the bandwidth of SDN control channels and their computing resources. We demonstrate the feasibility and effectiveness of these attacks with both theoretical analysis and experiments in a real SDN testbed. Finally, we develop a lightweight defense system that can be readily deployed in existing SDN controllers as a patch.