Sheikh `Abd Allah b. Fahd al-Hayd.
Normally, a person should wipe his hair when making ablutions, but if a woman is wearing a head scarf and needs to wipe over it out of necessity, such as there being difficulty in removing the scarf or fear of falling ill due to extreme cold, then she may wipe over the head covering itself.
If, however, there is no need for wiping over the scarf instead of the hair, then there is disagreement between scholars on the permissibility of doing so. There are some scholars who say it is lawful if the scarf is bound under her chin, because it was related from Umm Salamah that she used to wipe on her head covering.
And Allah knows best.
Source: Islam Today