security - Rails: How to implement protect_from_forgery in Rails API mode

Question

I have a Rails 5 API app (ApplicationController < ActionController::API). The need came up to add a simple GUI form for one endpoint of this API.

Initially, I was getting ActionView::Template::Error undefined method protect_against_forgery? when I tried to render the form. I added include ActionController::RequestForgeryProtection and protect_from_forgery with:exception to that endpoint. Which solved that issue as expected.

However, when I try to submit this form I get: 422 Unprocessable Entity ActionController::InvalidAuthenticityToken. I've added <%= csrf_meta_tags %> and verified that meta: csrf-param and meta: csrf-token are present in my headers, and that authenticity_token is present in my form. (The tokens themselves are different from each other.)

I've tried, protect_from_forgery prepend: true, with:exception, no effect. I can "fix" this issue by commenting out: protect_from_forgery with:exception. But my understanding is that that is turning off CSRF protection on my form. (I want CSRF protection.)

What am I missing?

UPDATE:

To try to make this clear, 99% of this app is a pure JSON RESTful API. The need came up to add one HTML view and form to this app. So for one Controller I want to enable full CSRF protection. The rest of the app doesn't need CSRF and can remain unchanged.

UPDATE 2:

I just compared the page source of this app's HTML form and Header with another conventional Rails 5 app I wrote. The authenticity_token in the Header and the authenticity_token in the form are the same. In the API app I'm having the problem with, they're different. Maybe that's something?

UPDATE 3:

Ok, I don't the the mismatch is the issue. However, in further comparisons between the working and non-working apps I noticed that there's nothing in Network > Cookies. I see a bunch of things like _my_app-session in the cookies of the working app.

Answer 1

Here's what the issue was: Rails 5, when in API mode, logically doesn't include the Cookie middleware. Without it, there's no Session key stored in a Cookie to be used when validating the token I passed with my form.

Somewhat confusingly, changing things in config/initializers/session_store.rb had no effect.

I eventually found the answer to that problem here: Adding cookie session store back to Rails API app, which led me here: https://github.com/rails/rails/pull/28009/files which mentioned exactly the lines I needed to add to application.rb to get working Cookies back:

config.session_store :cookie_store, key: "_YOUR_APP_session_#{Rails.env}"
config.middleware.use ActionDispatch::Cookies # Required for all session management
config.middleware.use ActionDispatch::Session::CookieStore, config.session_options

Those three lines coupled with:

class FooController < ApplicationController
  include ActionController::RequestForgeryProtection
  protect_from_forgery with: :exception, unless: -> { request.format.json? }
  ...

And of course a form generated through the proper helpers:

form_tag(FOO_CREATE_path, method: :post)
  ...

Got me a CSRF protected form in the middle of my Rails API app.