Jennifer Lopez -- Casper Smart's Dog Escapes ... Attacks Neighbor

Casper Smart's dog escaped from Jennifer Lopez's house and bit one of her neighbors ... leaving the woman with a broken hand.

Law enforcement sources tell TMZ ... Smart's boxer, named Bear was at JLo's home outside Los Angeles back in December ... when he got loose and clamped down on the neighbor's hand and arm.

Since the attack, Bear has been quarantined at Casper's nearby Woodland Hills home. Authorities are still trying to determine if the dog poses a danger to others.

We're told both of Casper's boxers still spend a lot of time at JLo's house ... since her kids love the dogs. It's unclear if JLo or Casper were at the house when the attack happened, but Animal Control gave Casper a citation for an unleashed dog ... since he's the owner.