Cloudflare Flexible SSL mode breaks Rails 5 CSRF

Putting this out there since I didn't find anything on StackOverflow or other places concerning this problem, which I'm sure I'm not the first to run into. CloudFlare is great, especially as a way to set-and-forget SSL on your site, along with all the other benefits you get. It acts as a proxy to your app, and if you set its SSL mode to Flexible then you don't have to have an SSL certificate setup on your server. This used to be a big deal when SSL certificates were expensive. (You could argue that since Let's Encrypt and free SSL certificates it's not worth using Flexible mode anymore.)

Anyway, I digress. The point of this TIL is that if you proxy https requests to http endpoint in Rails 5, you'll get the dreaded InvalidAuthenticityToken exception whenever you try to submit any forms. It has nothing to do with the forgery_protection_origin_check before action in ApplicationController.

The dead giveaway that you're having this problem is in your logs. Look for the following two lines near each other.

WARN -- : [c2992f72-f8cc-49a2-bc16-b0d429cdef20] HTTP Origin header ( didn't match request.base_url (  
FATAL -- : [c2992f72-f8cc-49a2-bc16-b0d429cdef20] ActionController::InvalidAuthenticityToken (ActionController::InvalidAuthenticityToken): 
Aug 13 18:08:48 pb2-production app/web.1: F, [2017-08-14T01:08:48.226341 #4] FATAL -- : [c2992f72-f8cc-49a2-bc16-b0d429cdef20]    

The solution is simple. Make sure you have working SSL and HTTPS on Heroku (or wherever you're serving your Rails application.) Turn Cloudflare SSL to Full mode. Problem solved.