Imaam an-Nawawee said in his Riyaadh-us-Saaliheen,  chapter 256:
“You should know that backbiting is permissible for specific legislated reasons not able to be reached except by it (backbiting), it is the following six reasons:
The First Reason: Is complaining of oppression, so it is permissible for the oppressed person to present his case to the Muslim ruler or Muslim judge or someone of similar stature and authority to assist in removing the oppression. He can say, ‘I have been wronged by such and such a person.’
The Second Reason: Is seeking assistance in changing evil (Munkar) and refuting the disobedient until they return to what is correct. So one can say to the one who has the ability to change the evil, ‘Such and such a person has done this; so advise him.’ Or something similar, and that he intends to reach this disobedient one by way of removing the evil, so if he intends other than that then it is not permissible.
The Third Reason: When one seeks a Fatwaa. So he says to the Mufti, ‘My father has oppressed me, or my brother, or my husband, or such and such has done this, does he have right to? And what is the way to sincerity (Ikhlaas) and obtaining my rights and repelling oppression?’ or something along the lines, so this is permissible by necessity. However it is better to say, ‘What do you say regarding a man, or person, or husband, if his condition is such and such?’ So he obtains his purpose without specification, but being specific is permissible, as we will mention in the Hadeeth of Hind, Allaah willing.
From it is the disparagement of the criticized people by way of the reliable and trustworthy witnesses. That is permissible by consensus of the Muslims; rather it is obligatory by necessity.
From it is when a man inquires about someone before marriage, or entering into a business partnership with someone, or safekeeping something with someone, or doing business with someone. It is obligatory upon the one advising not to conceal his condition rather he should mention the shortcomings he has with the sole intention of advising him.
From it is if one sees a person of Fiqh frequenting an innovator or a Faasiq and taking knowledge from him, and he fears that he may affect him (with his innovation or Fisq), then it is upon him to advise of his condition on the basis that he intends to advise. And this is where error may occur, the person who speaks of the individual could be envious and Shaytaan (may Allaah’s curse be upon him) may deceive him into thinking he is advising with compassion, it is necessary to be aware of this (type of Fitnah).
From it is one who has position but he does not establish it, whether it is because he is not fit for it, or he is one who is corrupt, or neglectful, or something similar to these examples. It is obligatory to mention that to the one in authority so as to remove him and replace him with one more befitting. He should make him aware of his condition so he can be dealt with and urge him to improve or be replaced with someone else.
The Fifth Reason: Is concerning the one who openly commits disobedience or innovation like openly drinking alcohol, taking the property of others, taking taxes not legislated or other issues of falsehood, then it is permissible to mention what he does publicly. It is not permissible to mention other faults he may have except those we have mentioned before.
The Sixth Reason: Is to describe someone. So if a man is known by a nickname such as “The Weak-eyed”, “The Lame”, “The Deaf one”, “The Blind one”, “The Cross-eyed”, and other similar names. It is permissible to describe him for identification but unlawful to indicate his deficiencies. If one can describe him another way then it is best.
 The current English translation of this chapter by Dar-us-Salaam is missing portions that deal with the people of innovation. Likewise the Spanish translation, “El Paso hacia el Paraíso” translated by Ahmad M Safi oddly enough, has omitted the entire text of the Noble Imaam’s introduction to this chapter!
Source: Riyaadh-us-Saaliheen chapter 256
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