Software-Defined Networking (SDN) is an emerging network architecture that provides programmable networking through a logically centralized controller. As SDN becomes more prominent, its security vulnerabilities become more evident than ever. Serving as the "brain"of a software-defined network, how the control plane (of the network) is exposed to external inputs (i.e., data plane messages) is directly correlated with how secure the network is. Fortunately, due to some unique SDN design choices (e.g., control plane and data plane separation), attackers often struggle to find a reachable path to those vulnerable logic hidden deeply within the control plane.In this paper, we demonstrate that it is possible for a weak adversary who only controls a commodity network device (host or switch) to attack previously unreachable control plane components by maliciously increasing reachability in the control plane. We introduce D2C2 (data dependency creation and chaining) attack, which leverages some widely-used SDN protocol features (e.g., custom fields) to create and chain unexpected data dependencies in order to achieve greater reachability. We have developed a novel tool, SVHunter, which can effectively identify D2C2 vulnerabilities. Till now we have evaluated SVHunter on three mainstream open-source SDN controllers (i.e., ONOS, Floodlight, and Opendaylight) as well as one security-enhanced controller (i.e., SE-Floodlight). SVHunter detects 18 previously unknown vulnerabilities, all of which can be exploited remotely to launch serious attacks such as executing arbitrary commands, exfiltrating confidential files, and crashing SDN services.