The most common site of leak in CSF rhinorrhoea is -
Sphenoid sinus
Frontal sinus
Cribriform plate
Tegmen typmani
Leakage of CSF is most commonly seen in the anterior cranial fossa at the level of the cribriform plate as it is extremely thin. (Ref: Diseases of nose ear and throat; Logan Turner; pg 28)
Nose and paranasal sinuses