Is it permissible for a wife to attach hair extensions to her very sparse hair, while wearing the hijab, and is this considered forbidden adornment?

The summary of the scholars' opinions on hair extensions is that they are forbidden if they involve impure hair or human hair. This applies to both married and unmarried women, whether with or without the husband's permission. This is based on the Prophet's (peace be upon him) saying: "May Allah curse the woman who affixes hair extensions and the one who has them affixed, and the woman who does tattoos and the one who has them done."

We advise the wife and others to abstain from adorning themselves with forbidden means and instead use permissible forms of adornment, and to remember the Prophet's (peace be upon him) saying: "Indeed, you will not abandon anything for the sake of Allah, the Mighty and Majestic, except that Allah will replace it with something better for you."