Backbiting (ghibah) is forbidden (haraam) in Islam. I remember some exceptions to this, where talking behind someone's back isn't considered ghibah, but I can't remember what they were.
For example, when your boss asks you about your opinion on your co-worker, is it OK to point their flaws? Are other situations which would be acceptable?
Backbiting is permissible only for valid reasons approved by Shari`ah. These reasons are as follows:
It is permissible for an oppressed person to speak before the judge or someone in a similar position of authority to help him or her establish his or her rights by telling him `so-and-so wronged me and has done such and such to me' etc.
It is permissible to seek somebody's assistance in forbidding evil and helping someone change his or her immoral conduct. One can say to the person who can offer such assistance, `so-and-so does such and such evil deeds. Can you exhort him?' etc. This is permissible as long as one intends to forbid evil. If, however, one intends something else apart from this, then this act becomes unlawful.
One who seeks legal verdict on a certain matter may point out the defaults of another person or relate something else. One in this case can say to the Mufti (religious scholar who issues verdicts): "My father or brother (for example) treated me unjustly. Can I get my right established?'' etc. This is permissible to say only if need be, but it is better to say `What do you think of someone who did such and such?' This does not mean, however, that naming the person in question is not permissible, as said in the following hadeeth:
`Aishah (May Allah be pleased with her) said: Hind, the wife of Abu Sufyan, said to the Prophet (sallallaahu ’alayhi wa sallam): Abu Sufyan is a niggardly man and does not give me and my children adequate provisions for maintenance unless I take something from his possession without his knowledge. The Prophet (sallallaahu ’alayhi wa sallam) said to her, "Take from his possessions on a reasonable basis that much which may suffice for you and your children.'' [Al-Bukhari and Muslim].
One who criticizes those who openly commit acts of disobedience, such as drinking wine, gambling, engaging in immoral habits, fornication, hypocrisy, and making mischief.
It is permissible to call into question the narrators of Hadith, and witnesses in the court when the need arises. It is also permissible to mention the bad qualities of somebody for marriage purposes in case an advice is sought. Also, if one has noticed that a "seeker of knowledge'' frequently goes to the gatherings of an innovator in religion and one fears that this "seeker of knowledge'' may be affected by this so-called scholar, then he must in this case give counsel to the "seeker of knowledge'' by telling him about the "innovator,'' etc.
Fatimah bint Qais (May Allah be pleased with her) said: I came to the Prophet (sallallaahu ’alayhi wa sallam) and said to him: "Muawiyah and Abul-Jahm sent me a proposal of marriage.'' The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, "Muawiyah is destitute and he has no property, and Abul-Jahm is very hard on women.'' [Bukhari and Muslim].
`Aishah (May Allah be pleased with her) said: The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, "I do not think that so-and-so understands anything of our Faith.'' [Al-Bukhari]
It is permissible to use names such as "Al-a`mash" which means 'the blear-eyed' to talk about people who are known by such names for the sake of identification and not for disparaging people and underestimating them. To identify them without resorting to such names is however better.
There are several Hadiths regarding this: Imam Al-Nawawi's Riyad-us-Saliheen
It's allowed when:
References: مصباح الشریعه و مفتاح الحقیقه and Forty Hadith
You should have your co-worker with you when you tell your boss what you think of him.
Ghibah [غيبة] is a word derived from the root word Ghaib [غيب] which means "That which is hidden".
In an authentic hadith, Abu Hurairah (May Allah be pleased with him) said:
The Messenger of Allah (sallallahu 'alaihi wa sallam) said, "Do you know what is ghibah (backbiting)?" The Companions said: "Allah and His Messenger know better." Thereupon he said, "Ghibah is talking about your (Muslim) brother in a manner which he dislikes." It was said to him: "What if my (Muslim) brother is as I say." He said, "If he is actually as you say, then that is ghibah; but if that is not in him, that is slandering."
وعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: “أتدرون ما الغيبة؟” قالوا: الله ورسوله أعلم قال: "ذكرك أخاك بما يكره" قيل: أفرأيت إن كان في أخي ما أقول؟ قال: "إن كان فيه ما تقول، فقد اغتبته، وإن لم يكن فيه ما تقول فقد بهته”. ((رواه مسلم))
