ASP.NET MVC5 OWIN Facebook authentication suddenly not working

Question

Update 2017!

The issue I had when I posted the original question has got nothing to do with the recent changes Facebook made when they forced everyo

Answer 1

The last Facebook upgrade was on 2015-02-09 (https://www.nuget.org/packages/Microsoft.AspNet.WebPages.OAuth/)

The latest version of the API at that point was version 2.2. Version 2.2 expired on the 25th of March 2017, which is coincidentally when the problem started. (https://developers.facebook.com/docs/apps/changelog)

I'm guessing Facebook probably automatically upgraded the API and now the MS OAUTH library is unable to parse the new response.

tldr: The Microsoft WebPages OAuth library is outdated (for FB at least) and you'll probably have to find another solution