Ferrari has been told by the FIA that the winglets that have been attached to the halo have to be removed after the Spanish Grand Prix, as the FIA received multiple complaints about it already.
The idea of attaching mirrors on the sides of the halo isn't the illegal part, but the Italian team also attached little winglets on top of them for aerodynamic gain. That's the part that other teams don't like.
“The FIA received letters of complaint,” German source Auto Motor und Sport reports.
“Ferrari has now been asked to bring a modified solution to Monaco.
“The mirror may still be attached to the Halo, but without aerodynamic elements.”
Other teams have also been working on attaching the mirrors to the halo itself, but Ferrari were the first ones to roll that update out. Renault has already said that they're currently working on mirror concepts, while Force India's Andy Green admitted that the gain of the winglets is very small, so they'll just copy the best one.