As a lot of sophisticated duties are being migrated to mobile phones, they are gradually becoming hot targets of hackers. Actually, during the past few years, It has appeared many malware targeting mobile phones and the situation is getting worse. Under this circumstance, we may ask a serious question: whether can those infected phones be organized to a botnet? In this paper, we present a design of such a botnet using Short Message Service (SMS) as its Command and Control (C&C) medium. We cover all the aspects of the botnet design including the stealthiness protection, the topology selecting and the botnet maintaining. Our simulations show that in our proposed SMS-based botnet a newly issued C&C message can be covertly propagated to over 90% of the total 20000 bots within 20 minutes based on a simple flooding algorithm. Moreover, in this process each bot sends no more than four SMS messages and the botnet is robust to both random and selective node failures. Thereby, we demonstrate that the proposed mobile botnet is indeed a serious threat on the security of the mobile computing environment. For this reason, we further explore several effective defense strategies against such a botnet. In doing so, we hope to be one step ahead of the hackers to discover and prevent this upcoming threat.