It could go down as one of the most bizarre goals in European history and it came in the 51st minute.
Karius comfortably collected a through pass from Toni Kross and tried to start a quick counter by underarming the ball out to team-mate Dejan Lovren.
However, he misjudged how near Benzema was, who only needed to stretch out a leg for the ball to rebound off and trickle miserably into the corner of the net.
The goal means Benzema finally breaks his final duck at the fourth occasion and moves Madrid one stop closer to their thirteenth European crown.