Byzantine attacks in collaborative spectrum sensing (CSS) systems of cognitive radio networks (CRNs) is a critical threat and has attracted considerable attention. However, in existing works, attacking strategies are generally modeled in a simple way, i.e., Single-Strategy, and the attackers can be detected easily. In practical CRNs, Byzantine attackers could be more sophisticated, and they could adjust their malicious actions according to the actions of normal nodes. In this paper, we propose a new type of attacks, i.e., Hybrid-Strategy attacks (HSAs), which are more flexible and destructive than Single-Strategy attacks. We analyze the properties of proposed HSAs and the limitations of existing attacker-detection methods when they encounter HSAs. Furthermore, we prove that the HSAs can avoid being detected by existing distance based detection algorithms when they evolve into stealthy attacks.