Single Page Application and CSRF Token

Client side (SPA)

You only need to grab the CSRF token once per session. You can hold onto it in the browser and send it on every (non-GET) request.

Rails will appear to generate a new CSRF token on every request, but it will accept any generated token from that session. In reality, it is just masking a single token using a one-time pad per request, in order to protect against SSL BREACH attack. More details at https://stackoverflow.com/a/49783739/2016618. You don't need to track/store these tokens.

Server side

I strongly suggest using Rails's protect_from_forgery directive rather than encoding the CSRF token in a header yourself. It will generate a different masked token per request.

You can certainly reproduce this yourself with not that much code, but I don't see why you'd need to.

Do you need CSRF protection with an API?

Yes! If you are authenticating with a cookie, you need CSRF protection. This is because cookies are sent with every request, so a malicious website could send a POST request to your site and perform requests on behalf of a logged in user. The CSRF token prevents this, because the malicious site won't know the CSRF token.

I don’t know what exact issue you are facing. But if you are getting CSRF issues in New Rails versions and need to include Rails CSRF tokens in ajax requests you can follow the steps below.

Recently I used Rails 5.1 application.

When using ajax calls to fetch some data from APIs I was getting CSRF token issues:

‘WARNING: Can't verify CSRF token authenticity rails’

The reason was

Rails 5.1 removed support for jquery and jquery_ujs by default, and added

//= require rails-ujs in application.js

It does the following things:

1. force confirmation dialogs for various actions;
2. make non-GET requests from hyperlinks;
3. make forms or hyperlinks submit data asynchronously with Ajax;
4. have submit buttons become automatically disabled on form submit to prevent double-clicking. (from: https://github.com/rails/rails-ujs/tree/master)

But it is not including the csrf token for ajax request by default. Beware of that. We have to explicitly pass it like:

$( document ).ready(function() {
  $.ajaxSetup({
    headers: {
      'X-CSRF-Token': Rails.csrfToken()
    }
  });
  ----
  ----
});

Note that in Rails 5.1 version, you get ‘Rails’ class in js, and can make use of the functions.

Update: If you are using Rails server side and other front end, you really don't want to use Rails provided CSRF tokens. Because Its really not matter which backend service you are using.

If your aim is to block CSRF, you need to set up CORS in backend, that is your Rails backend. Rails is now providing a separate file in initializer for this. You can mention which sites are allowed to send ajax requests to your backend.

Edit here:

config/initializers/cors.rb

If you want authentication, use basic auth, token auth Or JWT

I'll just answer the question. Full details are explained in this article: https://blog.eq8.eu/article/rails-api-authentication-with-spa-csrf-tokens.html

Given Rails is setting cookie for SPA

CSRF tokens are valid during the lifetime of the session. Therefore yes it's ok just to generate one CSRF token for the duration of session after login.

class LoginController < ApplicationController
  def create
    if user_password_match
      # ....
      cookie[:my_csrf_token]= form_authenticity_token
    end
  end
end

or you can refresh the cookie the same way as you are proposing

class ApplicationController < ActionController::Base

  after_action :set_csrf_cookie

  def set_csrf_cookie
    cookies["my_csrf_token"] = form_authenticity_token
  end

end

Your SPA just needs to read the cookie and set it as header.

Both are equally valid

or You can just provide the CSRF token as a login response and SPA will store it somewhere and use it in the X-CSRF-Token header:

curl POST  https://api.my-app.com/login.json  -d "{"email":'[email protected]", "password":"Hello"}"  -H 'ContentType: application/json'
# Cookie with session_id was set

# response:

{  "login": "ok", "csrf": 'yyyyyyyyy" }

# next request 

 curl POST  https://api.my-app.com/transfer_my_money  -d "{"to_user_id:":"1234"}"  -H "ContentType: application/json" -H "X-CSRF-Token: yyyyyyyyy"

Given Rails is accepting header only authentication

if you are not using cookie to send session_id, therefore your API is using just Authentication header for authentication. Then you don't need CSRF protection.

No session cookie no CSRF problems !

Example:

curl POST  https://api.my-app.com/login.json  -d "{"email":'[email protected]", "password":"Hello"}"  -H 'ContentType: application/json'
# No cookie/session is set

# response:
{ "login": "ok", "jwt": "xxxxxxxxxxx.xxxxxxxx.xxxxx" }

# Next Request:
curl POST  https://api.my-app.com/transfer_my_money  -d "{"to_user_id:":"1234"}"  -H "ContentType: application/json" -H "Authentication: Bearer xxxxxxxxxxx.xxxxxxxx.xxxxx"

Once again, this is only when you don't use cookies for user identification ! So CSRF is not an issue in this case but you still protect from Cross site scripting attack, make sure your communication is HTTPs only, etc...