Can't hack server sided games easily.
Fixed.
Even the most secure server-sided game has to expose some actions to the user, otherwise they wouldn't be able to respond. If there's any leeway in those responses then that's a (relatively) easy hack. If all they do is forward the user input to the server then you have a much harder time of it. If they're prepared to simulate the response of every single user's input every time they interact (which, for some games, could be pretty well constantly), then you're basically out of luck, and they're out a huge amount of processing power - as well as having a game that is extremely prone to lag (and DOS attacks). Although with turn-based games this would be viable.
Fortunately, most server-sided games are not built that way. It's noteworthy that it's impossible to fully server-side a game that requires code to execute on the user's computer; the best that could be achieved is streaming the relevant code as it's required, which would be a nightmare to set up and use. Since all flash games have bytecode, all flash games can be hacked. Whether it's a useful hack (see the above about leeway in responses) is another question, but any time they assume "We don't really have to check this is legit" is an opening.