Reversing an online Flash application is sometimes not needed if you can access the data inside the model directly.
When you're developing an online application that's running client-side with data server-side, you're faced with the localization of data. For clarity, let's assume you're developing it in a MVC design pattern. Basically, you'd want to put most of data on the server and only give the client a controller on it. The problem starts when you need high transfer rates and reactivity: you just can't go to the server and back for every tiny piece of data. That's when you need to have the model split between the server and the client. Either split or duplicated.
What I'm going to say may come as an obviousness for people used to security, but it may as well come as a shock for casual application developers: if some part of your data model is located on the client, you need not only do user input validation on the inputs from the controller but also on the inputs from the client-side model.
You can't trust the client's terminal to keep data stored in the model safe. So you need to protect it either through integrity or through server-side re-validation.
I just hacked into putting whatever score I wish for myself onto an online gaming platform, which inspired me to write this article. The same could be applicable for more critical applications.