Is backbiting an innovator permissible, whether he knows about it or not?

Backbiting is forbidden by the Qur’an, the Sunnah, and the consensus of the Ummah, but it may be permissible in certain situations. Among these is warning against an innovator by enlightening people about his innovation and protecting them from it. This warning must not exceed mentioning the traits that serve this purpose, while ensuring that the intention behind the advice is to seek the pleasure of Allah and to reform the person or prevent his harm from reaching Muslims. There is no difference in the permissibility of backbiting an innovator whether he is aware of his innovation or not, because the need for warning necessitates it.