How we learned to cheat at online poker: a story of software security
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That being said, there are still problems.
Several months ago the 2+2 poker community discovered people using "superuser" accounts on Absolute Poker, which could see everyone else's holecards. Fortunately, the perpetrator wildly abused his powers and was discovered in time.
And who knows, you could discover some zero day exploit that gets you the same thing. All you have to do is spend thousands of hours attempting something that you probably won't succeed at and even if you did, you'd probably be discovered quickly because of inconsistencies in your play.
It's called 1 in a trillion for a reason ;)
Instead, what you'd do is you would play preflop without the knowledge. Then you'd probably want to set it up such that you were only getting the info some relatively small % of the time postflop too. It would be enough to turn anyone who was a marginal winner into a large consistent one, but not so big of one as to be caught, especially with routine account switches.
I think I've since been proven right by the fact that Ultimate Bet and Absolute Poker still exist.
Unfortunately whether it was a winning bet was done on the server, and the communication was just sending the bet to the server, and the other way, the result to the player.