normalize domains with trailing slashes

CSP3 more explicitly calls this out:

> If path A consists of one character that is equal to the U+002F
> SOLIDUS character (/) and path B is empty, return "Matches".

A URL like `example.com/foo` will match a connect-src of `example.com`,
as well as `example.com/`, so having two connect-srcs listed like this
is redundant.

Author: keithamus

lib/secure_headers/headers/content_security_policy.rb

@@ -129,6 +129,7 @@ def minify_source_list(directive, source_list)
       else
         source_list = populate_nonces(directive, source_list)
         source_list = reject_all_values_if_none(source_list)
+        source_list = normalize_uri_paths(source_list)
 
         unless directive == REPORT_URI || @preserve_schemes
           source_list = strip_source_schemes(source_list)
@@ -151,6 +152,26 @@ def reject_all_values_if_none(source_list)
       end
     end
 
+    def normalize_uri_paths(source_list)
+      source_list.map do |source|
+        # Normalize domains ending in a single / as without omitting the slash accomplishes the same.
+        # https://www.w3.org/TR/CSP3/#match-paths § 6.6.2.10 Step 2
+        begin
+          uri = URI(source)
+          if uri.path == "/"
+            next source.chomp("/")
+          end
+        rescue URI::InvalidURIError
+        end
+
+        if source.chomp("/").include?("/")
+          source
+        else
+          source.chomp("/")
+        end
+      end
+    end
+
     # Private: append a nonce to the script/style directories if script_nonce
     # or style_nonce are provided.
     def populate_nonces(directive, source_list)

spec/lib/secure_headers/headers/content_security_policy_spec.rb

@@ -48,9 +48,17 @@ module SecureHeaders
         expect(csp.value).to eq("default-src * 'unsafe-inline' 'unsafe-eval' data: blob:")
       end
 
+      it "normalizes source expressions that end with a trailing /" do
+        config = {
+          default_src: %w(a.example.org/ b.example.com/ c.example.net/foo/ b.example.co/bar)
+        }
+        csp = ContentSecurityPolicy.new(config)
+        expect(csp.value).to eq("default-src a.example.org b.example.com c.example.net/foo/ b.example.co/bar")
+      end
+
       it "does not minify source expressions based on overlapping wildcards" do
         config = {
-          default_src: %w(a.example.org b.example.org *.example.org https://*.example.org)
+          default_src: %w(a.example.org b.example.org *.example.org https://*.example.org c.example.org/)
         }
         csp = ContentSecurityPolicy.new(config)
         expect(csp.value).to eq("default-src a.example.org b.example.org *.example.org")