Issue with Relative Path Causing CORS Error on Discourse Sites

Yt.w · October 27, 2024, 6:27am

The following code in lib/highlight_js.rb works on some domains but not others:

def self.path
    "/highlight-js/#{Discourse.current_hostname}/#{version SiteSetting.highlighted_languages}.js"
end

I have two Discourse sites. This code works on the domain www.abc.com, but it doesn’t work on efg.com. In the browser console, I see the following error:

formatter.js:383 Uncaught (in promise) TypeError: Failed to resolve module specifier '/highlight-js/efg.com/9797975efac87d28baa695ae13ca72ccaf5120f5.js'. The base URL is about:blank because import() is called from a CORS-cross-origin script.

After modifying self.path in highlight_js.rb as follows:

def self.path
    "https://#{Discourse.current_hostname}/highlight-js/#{Discourse.current_hostname}/#{version SiteSetting.highlighted_languages}.js"
end

the issue was resolved.

It appears that using a relative path like
import('/highlight-js/efg.com/9797975efac87d28baa695ae13ca72ccaf5120f5.js')
causes a CORS issue, while using an absolute URL,
import('https://efg.com/highlight-js/efg.com/9797975efac87d28baa695ae13ca72ccaf5120f5.js') , works fine.

NateDhaliwal · October 27, 2024, 8:22am

Odd that it works with abc.com but not efg.com, since the 2 different routes are completely different

Yt.w · October 27, 2024, 9:04am

I mean it works with www.abc.com but not efg.com, not sure why but it happens on my cases

simonk · October 28, 2024, 11:49am

Are you using a CDN on either of these sites? I have noticed that code highlighting is no longer working on my site, and I think it’s because of this:

In my case, my CDN is not returning an Access-Control-Allow-Origin header for the highlightjs file. I notice that Meta’s CDN does include that header, so I wonder what is different.

$ curl --silent -I https://d3bpeqsaub0i6y.cloudfront.net/highlight-js/meta.discourse.org/9797975efac87d28baa695ae13ca72ccaf5120f5.js | grep -i access-control
access-control-allow-origin: *
access-control-allow-methods: GET, HEAD, OPTIONS

However, those headers are not being served by the origin server:

$ curl --silent -I https://meta.discourse.org/highlight-js/meta.discourse.org/9797975efac87d28baa695ae13ca72ccaf5120f5.js | grep -i access-control
<no output>

As far as I can tell, Discourse is meant to add access-control headers to the highlightjs files:

github.com/discourse/discourse

app/controllers/highlight_js_controller.rb

27c20eeac


      
          # frozen_string_literal: true
          
          class HighlightJsController < ApplicationController
            skip_before_action :preload_json,
                               :redirect_to_login_if_required,
                               :redirect_to_profile_if_required,
                               :check_xhr,
                               :verify_authenticity_token,
                               only: [:show]
          
            before_action :apply_cdn_headers, only: [:show]
          
            def show
              no_cookies
          
              RailsMultisite::ConnectionManagement.with_hostname(params[:hostname]) do
                current_version = HighlightJs.version(SiteSetting.highlighted_languages)
          
                return redirect_to path(HighlightJs.path) if current_version != params[:version]
          
                # note, this can be slightly optimised by caching the bundled file, it cuts down on N reads

However, those headers are only applied if the request is a “CDN request”:

github.com/discourse/discourse

app/controllers/application_controller.rb

27c20eeac


      
            return unless mini_profiler_enabled?
            Rack::MiniProfiler.authorize_request
          end
          
          def check_xhr
            # bypass xhr check on PUT / POST / DELETE provided api key is there, otherwise calling api is annoying
            return if !request.get? && (is_api? || is_user_api?)
            raise ApplicationController::RenderEmpty.new if !request.format&.json? && !request.xhr?
          end
          
          def apply_cdn_headers
            if Discourse.is_cdn_request?(request.env, request.method)
              Discourse.apply_cdn_headers(response.headers)
            end
          end
          
          def self.requires_login(arg = {})
            @requires_login_arg = arg
          end
          
          def self.requires_login_arg

github.com/discourse/discourse

lib/discourse.rb

27c20eeac


      
          end
          
          mattr_accessor :redis
          
          def self.is_parallel_test?
            ENV["RAILS_ENV"] == "test" && ENV["TEST_ENV_NUMBER"]
          end
          
          CDN_REQUEST_METHODS ||= %w[GET HEAD OPTIONS]
          
          def self.is_cdn_request?(env, request_method)
            return if CDN_REQUEST_METHODS.exclude?(request_method)
          
            cdn_hostnames = GlobalSetting.cdn_hostnames
            return if cdn_hostnames.blank?
          
            requested_hostname = env[REQUESTED_HOSTNAME] || env[Rack::HTTP_HOST]
            cdn_hostnames.include?(requested_hostname)
          end
          
          def self.apply_cdn_headers(headers)

This only works if Discourse is configured with a separate host name for “CDN requests”.

simonk · October 28, 2024, 12:02pm

There is a setting, cdn_origin_hostname, which could perhaps be set to the normal discourse host name:

github.com/discourse/discourse

app/models/global_setting.rb

27c20eeac


      
          end
          
          # for testing
          def self.reset_s3_cache!
            @use_s3 = nil
          end
          
          def self.cdn_hostnames
            hostnames = []
            hostnames << URI.parse(cdn_url).host if cdn_url.present?
            hostnames << cdn_origin_hostname if cdn_origin_hostname.present?
            hostnames
          end
          
          def self.database_config
            hash = { "adapter" => "postgresql" }
          
            %w[
              pool
              connect_timeout
              socket

github.com/discourse/discourse

config/discourse_defaults.conf

27c20eeac


      
          
          # authorization key that will be included as a header in requests made by the
          # snapshots transporter to the URL specified above. The destination should
          # know this key and only accept requests that have this key in the
          # `Mini-Profiler-Transport-Auth` header.
          mini_profiler_snapshots_transport_auth_key =
          
          # recommended, cdn used to access assets
          cdn_url =
          
          # The hostname used by the CDN to request assets
          cdn_origin_hostname =
          
          # comma delimited list of emails that have developer level access
          developer_emails =
          
          # redis server address
          redis_host = localhost
          
          # redis server port
          redis_port = 6379

I think this would cause the is_cdn_request check to pass, but I don’t know if it would have any negative side-effects.

Yt.w · October 28, 2024, 12:48pm

Yes, I am using a CDN on efg.com, but when I access the js file it do return me a correct Access-Control-Allow-Origin, that quite confused me

simonk · October 29, 2024, 5:48pm

As a follow up, I set cdn_origin_hostname to the normal domain name of my Discourse instance, flushed the CDN cache, and now highlightjs is working again. I haven’t noticed any side effects…

cuo_wu · May 18, 2025, 11:34am

I meet this problem too. Any update?

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Issue with Relative Path Causing CORS Error on Discourse Sites

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