The legal age of drinking in the US is 21.
She was at a party hosted by singer Adam Lambert in a nightclub, where she was spotted clutching what looked like a bottle of coconut rum in Malibu Oct 31, reports femalefirst.co.uk.
The "Hannah Montanna" star, who was dressed like rapper Lil Kim's ensemble from the 1999 MTV Video Music Awards, attempted to conceal her face with her hand when the picture was clicked, but she failed to disguise her identity or the bottle.
Cyrus was also photographed locking lips with guitarist Benji Madden on the same night.
In pictures: Bend it like Mumbai yogini Natasha Noel
Photos: Varun, Taapsee, Jacqueline promote 'Judwaa 2' on 'Dance Plus 3'
Instagram's newest sensation: Check out these photos of Bruna Abdullah
Photos: This is how Chitrangda Singh looks without make-up
Photos: Malaika Arora parties wearing see-through lace dress