What is the ruling on attaching natural hair to increase its density for a woman who wears hijab, and does not intend it for enticement, and her hair is very thin with gaps, and she wishes to make it reach a normal density?

There is no harm in naturally increasing hair through permissible medications. However, attaching real hair from another human is a forbidden act of وصل (hair extension). The majority of jurists have ruled it forbidden, whether it is the hair of a man or a woman. They based their ruling on prophetic traditions that prohibit وصل and curse the woman who attaches hair extensions (al-wasilah) and the woman who has them attached (al-mustawsilah), and because it is forbidden to benefit from human hair and other human body parts due to their dignity.