Shaykh Muhammad ibn Uthaymeen (may Allaah have mercy on him) said: This action - Hajj without a mahram - is haraam because of the hadeeth of Ibn Abbaas (may Allaah be pleased with him) who said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say, when he was delivering a sermon, No woman should travel except with a mahram. A man stood up and said, O Messenger of Allaah, my wife has set out for Hajj, and I have signed up for such-and-such a military campaign. The Prophet (peace and blessings of Allaah be upon him) said: Go and do Hajj with your wife.
Narrated by al-Bukhaari, 3006; Muslim, 1341.
So it is not permissible for a woman to travel without a mahram. A mahram is a man whom she is forever forbidden to marry because of blood ties or for a permissible reason. This is also subject to the condition that he be an adult of sound mind. A minor child cannot be a mahram, and neither can one who is not of sound mind. The reason for a mahram's presence being required is so that he can look after her and protect her, so that she will not be bothered by those who do not fear Allaah and do not show mercy to the slaves of Allaah.
It makes no difference whether there are other women with her or not, or whether she is safe or not. Even if she goes with women from her own family and she is extremely safe, it is not permissible for her to travel without a mahram, because when the Prophet (peace and blessings of Allaah be upon him) commanded that man to go and perform Hajj with his wife, he did not ask whether there were other women with her or not, or whether she was safe or not. The fact that he did not ask about that indicates that it makes no difference. This is the correct view.
Some people nowadays take the matter lightly and allow women travelling alone in a plane without a mahram. This undoubtedly goes against the clear meaning of the texts. Travel by plane is like any other kind of travel and still involves danger.
If the mahram of a woman travelling by plane takes her to the airport, as soon as she reaches the departure hall, he has to leave her there, then she is on her own without a mahram. The plane may leave on time or it may be late. It may leave on time but then have to come back for some reason, or it may land in another airport which is not the one she was heading for, and so on. It may land in the airport she is heading for after the expected time for some reason. If it happens that it lands on time, the mahram who is supposed to meet her may be late for some reason, either because he overslept or because of traffic congestion, or because his car broke down, or for some other reason. And even if he is there on time and meets the woman, there may have been a man sitting next to her on the plane who would deceive her and may like her and she likes him.
The point is that women should fear Allaah and not travel for Hajj or for any other reason except with a mahram who is an adult of sound mind. And Allaah is the One Whose help we seek.
Reproduced from Islam QA