It’s real. Taken at a windy airport greeting ceremony. The photographers were just snapping away unaware of what they were shooting but numerous shots were quite revealing that she was wearing a thong that day. A few papers published and got a lot of complaints of tastelessness. She probably would have been better off with something else that would have prevented the wardrobe malfunction.
Weights in the hem of the dress. This never happened to Diana, folks! (And that photo of her legs through the thin skirt when she was still single doesn’t count!)