What is the ruling on a woman traveling without a mahram and without her husband's permission?

It is not permissible for a woman to travel except with a mahram, due to the Prophet's (peace be upon him) saying: "It is not lawful for a woman who believes in Allah and the Last Day to travel for a distance of one night unless she is accompanied by a man who is a mahram to her," and his saying: "A woman should not travel except with a mahram." It is also not permissible for her to travel without her husband's permission, even if she is with a mahram, unless the travel is for performing the obligatory Hajj or for an urgent necessity such as treating a chronic illness. In these two cases, she may travel even without her husband's permission, provided she finds a mahram to accompany her.