The leak is not expected to do too much damage as users downloading a dodgy copy of the game from a file-sharing site would need a modified Xbox console to play the game.
This would be a feat in itself as Microsoft began banning modified consoles from the Xbox Live network earlier this year.
The company said that the measure is designed to protect the majority of players from those who may have an unfair advantage through a modified console.
Another factor is that the Halo 3 file weighs in at over 6GB, making it a hefty download.
A few sites are blaming the leak on UK retailer Argos, which inadvertently released copies of the game ahead of the launch date after an " administrative error".
However, although Microsoft is none too pleased with Argos, the firm has said that it will not punish owners of a genuine copy of the game for logging on early to get in some game time.
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