Praise be to Allah.
It is haraam because the Prophet (peace and blessings of Allah be upon him) said: “No woman should travel without a mahram.” This was narrated by Ahmad from the hadeeth of ‘Abd-Allah ibn ‘Abbaas (may Allah be pleased with him) in his Musnad. Even if her work requires that of her, we say that if she cannot find a mahram then she should not travel.
