The leak comes courtesy of the Japanese Xbox One website, although the details and screenshots of the game have since been removed. Presumably Microsoft had intended for the details to be released at E3 2015 during their press conference, or at least closer to the date, but perhaps someone at Microsoft got a little trigger happy and published said details a little earlier than expected.
According to the details of the upcoming game, it will feature split-screen support for up to two players. It will also feature multiplayer mode where up to 24 gamers will be able to play at once, the return of Drivatars, and will also reportedly include 26 world-famous locations that gamers can race around in.
There is also promise of 1080p @ 60fps visuals, over 450 cars to choose from, and more. Forza 6 was announced earlier this year but details of the game were scarce at that time. No word on when we will see the release of the game, but presumably Microsoft will share those details at E3 2015, so be sure to check back with us then for more information!