diff --git a/lib/url_helper.rb b/lib/url_helper.rb
index fe9e62d28..df25e5463 100644
--- a/lib/url_helper.rb
+++ b/lib/url_helper.rb
@@ -9,6 +9,7 @@ class UrlHelper
   end
 
   def self.absolute(url, cdn = Discourse.asset_host)
+    cdn = "https:" << cdn if cdn && cdn =~ /^\/\//
     url =~ /^\/[^\/]/ ? (cdn || Discourse.base_url_no_prefix) + url : url
   end
 
diff --git a/spec/components/url_helper_spec.rb b/spec/components/url_helper_spec.rb
index e9f580f61..972f3daa5 100644
--- a/spec/components/url_helper_spec.rb
+++ b/spec/components/url_helper_spec.rb
@@ -30,6 +30,21 @@ describe UrlHelper do
 
   describe "#absolute" do
 
+    it "returns an absolute URL for CDN" do
+      begin
+        Rails.configuration.action_controller.asset_host = "//cdn.awesome.com"
+        expect(UrlHelper.absolute("/test.jpg")).to eq("https://cdn.awesome.com/test.jpg")
+
+        Rails.configuration.action_controller.asset_host = "https://cdn.awesome.com"
+        expect(UrlHelper.absolute("/test.jpg")).to eq("https://cdn.awesome.com/test.jpg")
+
+        Rails.configuration.action_controller.asset_host = "http://cdn.awesome.com"
+        expect(UrlHelper.absolute("/test.jpg")).to eq("http://cdn.awesome.com/test.jpg")
+      ensure
+        Rails.configuration.action_controller.asset_host = nil
+      end
+    end
+
     it "does not change non-relative url" do
       expect(UrlHelper.absolute("http://www.discourse.org")).to eq("http://www.discourse.org")
     end