There have been photos spreading around online showing Zac Efron in his birthday suit. The photos show Zac Efron in a hotel balcony in Sidney, Australia. Unfortunately the photos are fake, this according to Just Jared.com website. See the other photos in this link http://www.justjared.com/photos/zac-efron/2/ . The original photos show that he was wearing dark pants with his underwear sticking out at that time. The person who photoshopped the photo did a good job in convincing people that it was real.
There are speculations that this so called "nude" photos were recently released to draw more attention to Zac Efrons new movie "The Lucky One" which opens in theaters this April 20th.