This article accompanies the fable
Typically a debate over the authenticity of a Leonardo is fought over by academics. It goes like this: Carlo Pedretti (UCLA) declares it's a Leonardo and Martin Kemp (Oxford) says it isn't. The debate then drives other art critics up the wall.
In the case of this particular painting, we know that while many of Leonardo's paintings have been lost, we have two preparatory drawings that survive, as well as copies by his students. Salvator Mundi (Saviour of the World) was a familiar icon to Renaissance painters and many took a crack at it.
Below: before the restoration.
Another disputed artwork is the one below, a red chalk drawing that some regard as a self-portrait from around 1510-1515, while others say it isn't. The argument - heated at times - turns on whether Leonardo ever would have considered doing a self-portrait, given that there appear to be no others, whether it is by Leonardo (there seems to be more agreement here) and whether its popularity arose because of wishful thinking - because people wanted to believe that we had a portrait of Leonardo in old age.
The drawing is stored the Biblioteca Reale (Royal Library) in Turin.