All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Backbiting is to mention a deficiency of a brother in his absence. The Prophet said: "Backbiting is to say something about your brother that he does not like to be said." [Muslim]
The scholars stated that it is permissible to backbite if there is an apparent benefit like when being wronged, when asking for a religious ruling and when warning the Muslims about an evil that might occur and so on. However, one should be careful not exceed the need in doing so and be cautious not to do that as a way of revenge and so on.
Allaah knows best.