In the first half of the match at Hughenden, the touch judge reported Fox to the French referee Eric Derriere for headbutting Sean Lamont, and the Saints flanker was given a yellow card.
But Fox was again involved in another unsavoury incident in the second half and was not seen by either referee or touch judges. Video evidence suggested that Fox had again headbutted. This time the victim was Cammy Mather, the Glasgow flanker.
Fox attended an ERC citing hearing in Dublin on Monday after Glasgow had cited him for the second incident for which the ban was handed down.
Saints are considering appealing the decision.