The Java Card uses two components to ensure the security of its model. On the one hand, the byte code verifier (BCV) checks, during an applet installation, if the Java Card security model is ensured. This mechanism may not be present in the card. On the other hand, the firewall dynamically checks if there is no illegal access. This paper describes two attacks to modify the Java Card control flow and to execute our own malicious byte code. In the first attack, we use a card without embedded security verifier and we show how it is simple to change the return address of a current function. In the second attack, we consider the hypothesis that the card embeds a partial implementation of a BCV. With the help of a laser beam, we are able to change the execution flow.