Know that although backbiting is forbidden, it becomes permissible under certain circumstances when done for a beneficial reason. The authorization for doing it must be based on a valid and legitimate reason of which in its absence, its permissibility cannot be achieved. The authorization (making backbiting allowable) can be any one of the following six reasons:
- Oppression - It is permissible for the one who is oppressed to complain about his situation to the ruler or the judge or anyone else who holds authority or has the ability to grant him justice against his oppressor. He should say: "Such and such person wronged me" and "he did such thing to me", and "he coerced me in this manner" and so on.
- Seeking assistance in changing an evil and in returning a sinner back to what is correct - One should say to the individual whom he expects has the capability of putting a stop to the evil: "Such and such person did this, so I prevented him from it" or something to that effect. His objective should be to look for a way to ultimately put an end to the evil. If he does not intend that as his goal, then it is forbidden (for him to mention it).
- Seeking a fatwa (religious ruling) - One should do this by saying to the muftee (scholar capable of issuing a fatwa): "My father" or "my brother" or "such and such person wronged me in this way." "Does he have the right to do so?" "How shall I go about putting an end to it and obtain my right while repelling oppression from myself?" and so on. Likewise, one may say: "My wife did such and such to me" or "my husband did such and such a thing" and so on. This is permissible due to the necessity for it, however, to be more cautious, it is better for one to say: "What do you say about a man who has done such and such thing?" or "concerning a husband" or "concerning a wife who did such and such" (without saying “my”), etc.
By doing this, the goal is achieved without having to resort to specifying anyone. However, specifying an individual by name is permissible (in this circumstance), based on the hadeeth of Hind (رضي الله عنها), which we shall mention later, by the Will of Allaah, in which she told Allaah’s Messenger: "Indeed, Abu Sufyaan (her husband) is a stingy man." And the Messenger of Allaah (صلى الله علیه وسلم) did not forbid her from saying this.
- Warning and Advising the Muslims against Evil - There are several perspectives to this, of which one is: Declaring someone unreliable in the field of narrating hadeeth and giving testimony. This is permissible to do, according to the Ijmaa' (consensus of the Muslim scholars). Rather, it becomes obligatory due to its necessity. Another case is when an individual desires to enter into a relationship with another person either through marriage, business, the consignment of property, the consigning of something to him or any other of the daily affairs. It is obligatory on you to mention to that individual what you know about the person he wants to get involved with, with the intention of advising him.
If your objective can be achieved by simply saying: "It is not good for you to engage with him in business transactions” or “in a relationship through marriage" or by saying: "You should not do this" or anything similar to that, then adding more to this, such as by mentioning his bad characteristics is not permissible. And if the objective cannot be reached, except by specifically explaining that person’s condition to him, then you may mention that to him in detail. Another case is when you see someone buying a product from an individual who is known for stealing or fornicating or drinking or other than these. It is then upon you to inform the buyer of this, on the count that he is not knowledgeable of it already. And this case is not specified to this example only. Rather, it also applies when you have knowledge that the commodity that is being traded is defective. It is then obligatory upon you to clarify this matter to the buyer, if he does not know of it.
Another case is when you see a student going to an innovator or a deviant, seeking to attain knowledge from him, and you fear that may affect the student. In that situation, you must advise him about the condition of that innovator, on the condition that your intention only be for the sake of advising. And this is something in which regard many people fall into error, for perhaps the person speaking may do this (advising) because he is jealous (of the person he is warning against). Or perhaps the Devil may deceive him about this matter, causing him to believe that what he is doing is advising and showing compassion, so he believes this.
One last case is when a person has some leadership role, which he does not fulfill properly either because he is not fit for it or because he is a sinner or neglectful, etc. So in this case, one must mention this to those who have general leadership over this person, so that he can be removed and someone fit can be put in charge. Or those who have charge over him can know this about him so that they can deal with him accordingly and not be deceived by him, and so that they can make the right efforts to encourage him to be upright or to replace him.
- When one openly exposes his acts of evil or his innovation - An example of this is when someone has openly exposed his consumption of alcohol, or his illegal confiscation of people’s money and raising of their taxes unjustly and his usurping command wrongfully. It is thus permissible for one to talk about what that individual has made public. But it is forbidden to mention any of his other defects, unless they fall under one of the categories, which we have mentioned that backbiting is permissible.
- Defining someone - If someone is known to the people by his nickname, such as “the bleary eyed one”, “the one who limps”, “the deaf guy”, “the blind guy”, “crosseyed”, “flat-nosed”, and other than that, then it is permissible to particularize him as such, with the aim of identifying him. However, it is forbidden to apply that to him, when one’s intention is to degrade him. If he can be identified with another (more appropriate) type of name, then that is more preferable. These are the six cases, in which the scholars have stated that backbiting is permissible, if it is done in accordance to the guidelines we mentioned above.
From those who have reported similar to what we have stated is Abu Haamid Al-Ghazaalee in his book “Al-Ihyaa”, as well as other scholars. The evidences for the permissibility of backbiting can be found in authentic and well-known ahaadeeth. Furthermore, there is an agreement of the scholars concerning the allowance of backbiting in these six cases.
It is reported in the Saheehs of Al-Bukhaaree and Muslim that 'Aa'ishah (رضي الله عنها) said:
"A man sought permission of the Prophet (صلى الله علیه وسلم) to enter (his house), so he said: 'Permit him to enter, and what an evil brother to (his) relatives he is.' " 
Al-Bukhaaree uses this hadeeth as evidence for the permissibility of backbiting the people of mischief and doubts.
Ibn Mas'ood (رضي الله عنه) narrated:
"The Messenger of Allaah (صلى الله علیه وسلم) divided a portion (of war booty amongst the people), so a man from the Ansaar said: 'I swear by Allaah, Muhammad did not intend the face of Allaah by this (i.e. he was not fair).' So I went to Allaah’s Messenger and informed him of this. His face changed (i.e. he became mad) and said: 'May Allaah have mercy on Moosaa. He was indeed abused with greater than this, but he was patient.' " 
In some of the reports of the hadeeth, Ibn Mas'ood said:
"I said: I will not raise another hadeeth to him again, after this."
Al-Bukhaaree uses this hadeeth as proof that a person is allowed to inform his brother of what is being said about him.
'Aa'ishah (رضي الله عنها) reported that Allaah’s Messenger (صلى الله علیه وسلم) once said:
"I do not think that this person and that person know anything at all about our Religion." 
Al-Laith Ibn Sa’ad, one of the narrators of the hadeeth's chain said:
"They were two individuals from among the hypocrites (at his time)."
Zayd Ibn Arqam (رضي الله عنه) reported:
"We set out on a journey with the Prophet and the people suffered great difficulty (due to a lack of provisions). So 'Abdullaah Ibn Ubay  said to his companions: 'Don't spend on those who are with Allaah's Messenger so that they may disperse and go away from him.' He said: 'If we return to Madeenah, surely, the more honorable will expel the lowly ones from it. So I went to the Prophet (صلى الله علیه وسلم) and informed him of that. He sent for 'Abdullaah Ibn Ubay and asked him, but 'Abdullaah Ibn Ubay swore that he did not say so. So the people said: 'Zayd told a lie to Allaah's Messenger.' And what they said distressed me very much. Later Allaah revealed the confirmation of my statement in His saying; 'When the hypocrites come to you...' [Surah Al-Munafiqeen]" 
Also there is the hadeeth of Hind (رضي الله عنها), the wife of Abu Sufyaan, in which she said to the Prophet:
"Indeed, Abu Sufyaan is a stingy man." 
And also the hadeeth of Faatimah Bint Qays (رضي الله عنها), when the Prophet (صلى الله علیه وسلم) said to her (with regard to her accepting marriage proposals from two suitors):
"As for Mu'awiyah, then he is utterly poor. And as for Abu Jahm, then he does not cease to remove the stick from his shoulder (i.e. he beats his wives)." 
 Saheeh – Reported by Al-Bukhaaree (10/471 of al-Fath) and Muslim (2591).
 Saheeh – Reported by Al-Bukhaaree and Muslim and its checking has preceded.
 Saheeh – Reported by Al-Bukhaaree (10/485 of al-Fath).
 Translator’s Note: He was the leader of the hypocrites in Madeenah. Upon his death, Allaah revealed verses commanding the Prophet (صلى الله علیه وسلم) not to pray the funeral prayer over him.
 Saheeh – Reported by Al-Bukhaaree (8/664 and 646-648 of al-Fath) and Muslim (2772).
 Saheeh – Reported by Al-Bukhaaree (9/504 of al-Fath) and Muslim (1714).
 Saheeh – Reported by Muslim (1480).