Nowadays, we use mobile devices in all activities such as communication, play, surf on the internet, shop online and banking transactions. But the applications used do not always comply with the security requirements on mobile environment and this can cause vulnerabilities allowing attackers to take control of the phone and to steal some users' private data. That is why, it is important to take a look at the security of mobile applications especially of financial institutions. In this paper, we will present some security issues of android applications. We will also make a reverse engineering of an Android banking application, then a static analysis of its code to detect its weaknesses. After that, we will insert a malicious code that will help us not only to take control of the smartphone but also to make a DDOS attack on a simulated bank server. Finally, we will propose some countermeasures.