At least Wechat had pushed two things powerfully, one is QR Code, the other one is HTML5.
You are at the mercy of the source of the QR code, with no real way of being able to validate it before scanning.
In any case, how are QR codes different from text URLs? Do you check the destination of every web link and/or button before clicking? Do you disable JS so that you can be sure that the URL displayed in the status bar isn't different from the one which will really be launched?
Like this: http://i.imgur.com/mlIr3QP.jpg