diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index b26b81c939f..6ad17dfbce5 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -65,6 +65,12 @@ module ApplicationHelper
       if GlobalSetting.cdn_url
         path = path.gsub(GlobalSetting.cdn_url, GlobalSetting.s3_cdn_url)
       else
+        # we must remove the subfolder path here, assets are uploaded to s3
+        # without it getting involved
+        if ActionController::Base.config.relative_url_root
+          path = path.sub(ActionController::Base.config.relative_url_root, "")
+        end
+
         path = "#{GlobalSetting.s3_cdn_url}#{path}"
       end
 
diff --git a/lib/file_store/s3_store.rb b/lib/file_store/s3_store.rb
index b2f9c20988b..076ea2171ab 100644
--- a/lib/file_store/s3_store.rb
+++ b/lib/file_store/s3_store.rb
@@ -91,7 +91,7 @@ module FileStore
       return url if SiteSetting.Upload.s3_cdn_url.blank?
       schema = url[/^(https?:)?\/\//, 1]
       folder = @s3_helper.s3_bucket_folder_path.nil? ? "" : "#{@s3_helper.s3_bucket_folder_path}/"
-      url.sub("#{schema}#{absolute_base_url}/#{folder}", "#{SiteSetting.Upload.s3_cdn_url}#{Discourse.base_uri}/")
+      url.sub("#{schema}#{absolute_base_url}/#{folder}", "#{SiteSetting.Upload.s3_cdn_url}/")
     end
 
     def cache_avatar(avatar, user_id)
diff --git a/spec/components/file_store/s3_store_spec.rb b/spec/components/file_store/s3_store_spec.rb
index 99e46fe4ccc..49e72cf954b 100644
--- a/spec/components/file_store/s3_store_spec.rb
+++ b/spec/components/file_store/s3_store_spec.rb
@@ -285,17 +285,18 @@ describe FileStore::S3Store do
 
   describe '.cdn_url' do
 
-    it 'uses the correct path' do
-      url = "//s3-upload-bucket.s3-us-west-2.amazonaws.com/livechat/original/1X/2252ae522257fc537351e47bbdd34698936b6c38.jpeg"
-      expect(store.cdn_url(url)).to end_with('/livechat/original/1X/2252ae522257fc537351e47bbdd34698936b6c38.jpeg')
-    end
-
     it 'supports subfolder' do
-      SiteSetting.s3_upload_bucket = 's3-upload-bucket/livechat/community'
-      GlobalSetting.stubs(:relative_url_root).returns('/community')
+      SiteSetting.s3_upload_bucket = 's3-upload-bucket/livechat'
+      SiteSetting.s3_cdn_url = 'https://rainbow.com'
+
+      # none of this should matter at all
+      # subfolder should not leak into uploads
+      global_setting :relative_url_root, '/community'
       Discourse.stubs(:base_uri).returns("/community")
-      url = "//s3-upload-bucket.s3-us-west-2.amazonaws.com/livechat/community/original/1X/2252ae522257fc537351e47bbdd34698936b6c38.jpeg"
-      expect(store.cdn_url(url)).to end_with('/livechat/community/original/1X/2252ae522257fc537351e47bbdd34698936b6c38.jpeg')
+
+      url = "//s3-upload-bucket.s3.amazonaws.com/livechat/original/gif.png"
+
+      expect(store.cdn_url(url)).to eq("https://rainbow.com/original/gif.png")
     end
   end
 
diff --git a/spec/helpers/application_helper_spec.rb b/spec/helpers/application_helper_spec.rb
index 69a50d2ca52..06553a4fde4 100644
--- a/spec/helpers/application_helper_spec.rb
+++ b/spec/helpers/application_helper_spec.rb
@@ -23,6 +23,15 @@ describe ApplicationHelper do
         set_env "COMPRESS_BROTLI", "1"
       end
 
+      after do
+        ActionController::Base.config.relative_url_root = nil
+      end
+
+      it "deals correctly with subfolder" do
+        ActionController::Base.config.relative_url_root = "/community"
+        expect(helper.preload_script("application")).to include('https://s3cdn.com/assets/application.js')
+      end
+
       it "returns magic brotli mangling for brotli requests" do
 
         helper.request.env["HTTP_ACCEPT_ENCODING"] = 'br'
diff --git a/test/javascripts/lib/discourse-test.js.es6 b/test/javascripts/lib/discourse-test.js.es6
index 9627eafdbb9..94498a16462 100644
--- a/test/javascripts/lib/discourse-test.js.es6
+++ b/test/javascripts/lib/discourse-test.js.es6
@@ -9,3 +9,18 @@ QUnit.test("getURL on subfolder install", assert => {
     "relative url has subfolder"
   );
 });
+
+QUnit.test("getURLWithCDN on subfolder install with S3", assert => {
+  Discourse.BaseUri = "/forum";
+
+  Discourse.S3CDN = "https://awesome.cdn/site";
+  Discourse.S3BaseUrl = "//test.s3-us-west-1.amazonaws.com/site";
+
+  let url = "//test.s3-us-west-1.amazonaws.com/site/forum/awesome.png";
+  let expected = "https://awesome.cdn/site/forum/awesome.png";
+
+  assert.equal(Discourse.getURLWithCDN(url), expected, "at correct path");
+
+  Discourse.S3CDN = null;
+  Discourse.S3BaseUrl = null;
+});