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Actress Kate Bosworth threw a little party to at her house this past Saturday to celebrate the success of her new movie 21, which is #1 at the box office!

Unfortunately the party wasn't all grins 'n giggles - a guest at the party tells X17online that Kate's dog attacked her 21 co-star, Aaron Yoo!

Our source says the dog bit Yoo's face, and that the actor was sent to the hospital for stitches!

We're still waiting on comments from both Aaron's and Kate's publicists; we hope he's okay!