Disable content security policy for mailer previews

[LilyReile]

Rails 5.2 introduced a default content_security_policy block to Rails::ApplicationController. Rails::ApplicationController is required by the MailersController, but any app-defined application_controller is not loaded. This means that regardless of CSP configuration in an app-defined application_controller or content_security_policy.rb initializer, the default CSP values will be included in the response headers for mailer previews.

These default CSP header values break certain middleware (e.g., guard-livereload, profilers). To my knowledge there is no benefit to supporting CSP in mailer previews, so maybe we can just disable them there?

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[LilyReile]

@pixeltrix tagging you since I believe you implemented CSP

[sikachu]

I have a question — should this problem be fixed environment-wise (so, say, we disable CSP for development) or is doing controller-based the right thing?

[LilyReile]

@sikachu Do you mean disabling CSP for development across the whole application, or just for the mailer previews? Some teams rely on catching CSP errors during development by having their configuration match on dev and prod.

Presently even if the configuration's script_src/style_src is compatible with the assets requested by the mailer previews, the settings for those fields will be overridden by the default block in Rails::ApplicationController.

[rylanb]

I think it would be good to do unless Rails.env.production? for the disable of CSP you added? (But maybe I'm misunderstanding)

We're running into this locally, too.

[LilyReile]

@rylanb Well, mailer previews are not normally used in production. If for some reason you are, I guess it wouldn't hurt. Here's the monkey patch we're using as a stopgap:

# until this issue is resolved by rails team
# https://github.com/rails/rails/pull/33483
module MonkeyPatches
  require "rails/application_controller"
  class Rails::MailersController < Rails::ApplicationController
    content_security_policy(false)
  end
end

[rylanb]

Yeah, totally. Seems like maybe just disabling it for mailers is fine. I can't see how the mailers_controller would need the CSP ... but maybe good if user gen-ed content ended up being rendered / included?