This work focuses on: (1) understanding the impact of sinkhole attacks on tree-based routing topologies in Wireless Sensor Networks (WSNs), and (2) investigating cryptography-based strategies to limit network degradation caused by these attacks. This work is particularly important as WSN protocols that construct a fixed routing topology may be significantly affected by malicious attacks. Furthermore, considering networks deployed in a difficult to access geographical region, building up resilience against such attacks rather than detection is expected to be more beneficial. We thus first provide a simulation study on the impact of malicious attacks based on a diverse set of parameters, such as the network scale and the position and number of malicious nodes. Based on this study, we propose a single but very representative metric for describing this impact. Second, we present the novel design and evaluation of two \emph{simple and resilient} topology-based reconfiguration protocols that broadcasts cryptographic values. The results of our simulation study show that our reconfiguration protocols are practical and effective in improving resilience against sinkhole attacks, even in the presence of some collusion.