When backbiting is allowed

The definition of backbiting is to say something about a muslim that he dislikes, in his absence. It is allowed to backbite on six different occasions:

1. To complain about transgression and oppression. If someone wrongs you, you may complain to the judge, police, or anyone who has authority and is able to bring justice to the person who wronged you.
2. To seek help in order to remove some evil. If you see an evil and know someone who has the authority to remove it, you may go to him and say for instance; ” So and so is perpetrating such and such evil; please go and stop him”. Your purpose and intention should be to eradicate the evil; otherwise (if your intention is, for instance to put down a person you don`t like), you are perpetrating a haram (forbidden) deed.
3. To seek a Islamic legal ruling from a scholar. When necessary, you may go to a scholar an speak about how someone from your family, such and such brother or person wronged you, in order to learn from him what course of action you should take in dealing with your situation. You can, however, show some caution, which is better, yet still fulfill your purpose by not mentioning names.
4. To warn and advise muslims about evil. For example when a witness is brought before a judge, people can testify against the witness if he is known to have an untrustworthy character.
5. To warn people about someone who openly flaunts his evil, regardless of whether that evil is an innovation or a wicked deed. If someone openly drinks alcohol or if he cheats people of their rights openly and with impunity, you may warn people about him, but only about those of his evils that he performs publicly, you may not mention those of evils that he perpetrates when alone, unless any of the other five categories discussed here applies to him.
6. To identify a person. By saying the blind one, the deaf one, the mute one.  If your intention is to clarify the identity of the person you are talking about. Otherwise if your intention is to belittle him, it is haram (forbidden). Similarly, if you can avoid to use these kind of words  and still make clear his identity, it is better.

Scholars such as An-Nawawee have mentioned these six reasons as being justifiable reasons for backbiting; there is consensus about most of them, and they are based on authentic, well-known hadeeth.

Extract taken out of  ” the book of manners”.