CNN's article on this story also includes a solid video that goes into the background of the report and its sources, distribution and timeline
here.
One tidbit I got from it is that
the former MI-6 agent and his sources for the report have already been vetted by the FBI. And that it originated in Trump opposition research initiated by both Republicans and Democrats.
Also, McCain didn't receive this information until after the election. Harry Reid had already had this information and made a big stink about it in October publicly demanding FBI Director Comey to make these allegations available to the voters before the election.
Its a good watch to see why this report is making headlines now, and why the heads of all of the intelligence agencies are taking it seriously.
Edit: Mark Armbinder of USA Today also
tweeted that the 35 page dossier (that Buzzfeed released) is not the only source of information used in the two page summary provided to Obama, Trump and the Gang of Eight (the senior leaders of Congress for both parties).