It is not wise to try and fit anything to a soffit board, as they are not fixed in such a way as to be able to carry weight. In fact, nothing shoulde be fitted to either soffit or facia boards. Fix all items to the house wall.
also, if your security light is halogen then it will get very hot. if in close proximity to the plastic soffit it could melt, burn or at least warp. better to fix it to the wall.
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