There is no legal requirement that candidates be given access to security briefings, it has been done to ease transitions between Presidents. Given the fact that the possibility exists that a foreign government is trying to meddle in the current election and that members of some campaign staffs have actually received monies from foreign entities, this practice should either be stopped or have conditions attached.
For example - In order to receive intelligence briefings, an order should be issued that the candidates must have passed the same sort of screening that anyone else would to have access to that information. Also, the candidate must publish at least the past five years of tax returns so that all financial relationships can be investigated, too.