In fact, cardboard is one of the best brown materials you can add to your pile.
Add extra water so that it will release the carbon and be a great benefit to compost that you are using with other organic waste.
Make sure there are not extra packaging, films, tapes or painting on the cardboard though! Ink is fine, nothing with glossy coating.
Satin-feel finish cardboards are made of powdered clay which are compostable.
Laminated cardboard is covered in plastic and can't go into the pile.