Normally, Cystic Fibrosis Transmembrane Regulator (CFTR) gene transports chloride ions out of the cell by active transport. In cystic fibrosis, a mutation alters the tertiary structure of CFTR.
This means that there's a defect with the secretion of chloride. It has a defect with a membrane protein that normally serves as a channel for the movement of chloride ions across the membrane.
In cystic fibrosis sweat ducts, the Cl- conductance is virtually abolished because CFTR is the only apical pathway for chloride: no other anion channels appear to be in the duct.
The sodium conductance also seems to be low, leading to the hypothesis that CFTR activates ENaC in the sweat duct.
As per Stanford.edu, when Na+ attempts to flow out of a CF duct through remaining sodium-selective pathways, it is unaccompanied by Cl- and so it creates an excess of negative charge in the duct that attracts Na+ and prevents its further absorption. The net result is that very little NaCl is reabsorbed, resulting in a high salt content in CF sweat.
Learn more about Cystic Fibrosis (CF) by checking these 5 fast facts from Emilysentourage.org.