Zac Efron — Oops! I Just Bought A Raging Party Palace

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For a guy coming off of rehab, Zac Efron couldn”t have picked a worse home to buy … because his new crib is well-known as one of the biggest Hollywood party pads in L.A. …  and we have the pics to prove it.

A source tells our sources … just last year, Sean Kingston was living large bachelor style in the house … and says it was a pool party house where “some of the most ungodly events” went down.

We”re told … Sean rented the place for 45k a month … and threw some wild ass parties for his friends, including Justin Bieber, Chris Brown, Rihanna and Soulja Boy

According to our source … Sean”s ragers were so off the hook … guests had to sign a non-disclosure agreement — and then be photographed holding the piece of paper — before they could ever get past the door. So you know there was some crazy s**t.

Zac just dropped $3.9 mil on this place. Just hope it”s not haunted … by ghosts of parties past.