Enforce protocol allowlisting for image and image-set CSS funcs

https://drafts.csswg.org/css-images-4/ introduces `url()`-like
functionality in new `image` and `image-set` functions. This commit
adds enforcement of protocol allowlisting for these functions.

Note that `image-set` is already widely supported, whereas `image`
is not yet.

lib/sanitize/css.rb

@@ -272,6 +272,10 @@ def property!(prop)
             return nil unless valid_url?(child)
           end
 
+          if name == 'image-set' || name == 'image'
+            return nil unless valid_image?(child)
+          end
+
           combined_value << name
           return nil if name == 'expression' || combined_value == 'expression'
         end
@@ -345,4 +349,27 @@ def valid_url?(node)
     false
   end
 
+  # Returns `true` if the given node (which is an `image` or `image-set` function) contains only strings
+  # using an allowlisted protocol.
+  def valid_image?(node)
+    return false unless node[:node] == :function
+    return false unless node.key?(:name) && ['image', 'image-set'].include?(node[:name].downcase)
+    return false unless Array === node[:value]
+
+    node[:value].each do |token|
+        return false unless Hash === token
+
+        case token[:node]
+          when :string
+            if token[:value] =~ Sanitize::REGEX_PROTOCOL
+              return false unless @config[:protocols].include?($1.downcase)
+            else
+              return false unless @config[:protocols].include?(:relative)
+            end
+          else
+            next
+        end
+      end
+  end
+
 end; end

test/test_sanitize_css.rb

@@ -29,6 +29,12 @@
           "background: url('ht\\tp://example.com/http.jpg')",
           "background: url(https://example.com/https.jpg)",
           "background: url('https://example.com/https.jpg')",
+          "background: image-set('relative.jpg' 1x, 'relative-2x.jpg' 2x)",
+          "background: image-set('https://example.com/https.jpg' 1x, 'https://example.com/https-2x.jpg' 2x)",
+          "background: image-set('https://example.com/https.jpg' type('image/jpeg'), 'https://example.com/https.avif' type('image/avif'))",
+          "background: image('relative.jpg');",
+          "background: image('https://example.com/https.jpg');",
+          "background: image(rtl 'https://example.com/https.jpg');"
         ].each do |css|
           _(@default.properties(css)).must_equal ''
           _(@relaxed.properties(css)).must_equal css