In what circumstances is backbiting permissible?

Question:
In whose case is backbiting permissible? And is the hadith: authentic?

Answer:
The hadith is not authentic, as far as I know.
But the scholars mentioned six things in which backbiting is permissible:
When one displays open sinfulness.
And likewise in seeking a legal ruling.
And likewise in complaining of injustice.
And likewise in criticizing witnesses and narrators.
All these matters are without blame.
And likewise in sincere advice; when someone seeks your advice and consults you.
So these six matters do not involve backbiting: when criticizing witnesses, when criticizing narrators, when the one seeking a ruling consults you about such and such: a woman consults you about taking from her husband's wealth because he is stingy and does not give her her rights, or she consults you about a person who wronged her: should she sue him? Should she take from his wealth in exchange for her right? And the like of that.
So if a person displays open sinfulness, there is no backbiting regarding him: if he displays shaving the beard, and a person says, "This so-and-so has shaved his beard," it is not backbiting. And if he lets his garments hang below the ankles, and some people say about him that he is one who lets his garments hang, it is not backbiting; because he displayed it. He is the one who displayed the sin, he is the one who exposed himself, so he is not called one who is backbitten—may Allah keep us safe.