Equipped creature has "{1}, {T}, Sacrifice Ninja's Kunai: Ninja's Kunai deals 3 damage to any target."
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)
If a player gains control of a creature equipped with an opponent's Ninja's Kunai without also gaining control of the Equipment, they cannot activate the ability it grants the equipped creature because you cannot sacrifice permanents you don't control.