Merge branch 'bug/svg_sanitizer' into 'master'

Improve SVG sanitizer to handle namespaced attributes

* Small refactor in the SVG sanitizer
* Enable already whitelisted namespaced attributes to be allowed
* Disable `xlink:href`to reference any external resource

Fixes #18100 

See merge request !4427

1 files changed, 35 insertions, 13 deletions

diff --git a/lib/gitlab/sanitizers/svg.rb b/lib/gitlab/sanitizers/svg.rb
index 5e95f6c0529..8304b9a482c 100644
--- a/lib/gitlab/sanitizers/svg.rb
+++ b/lib/gitlab/sanitizers/svg.rb
@@ -12,23 +12,45 @@ module Gitlab
         def scrub(node)
           unless Whitelist::ALLOWED_ELEMENTS.include?(node.name)
             node.unlink
-          else
-            node.attributes.each do |attr_name, attr|
-              valid_attributes = Whitelist::ALLOWED_ATTRIBUTES[node.name]
-
-              unless valid_attributes && valid_attributes.include?(attr_name)
-                if Whitelist::ALLOWED_DATA_ATTRIBUTES_IN_ELEMENTS.include?(node.name) &&
-                    attr_name.start_with?('data-')
-                  # Arbitrary data attributes are allowed. Verify that the attribute
-                  # is a valid data attribute.
-                  attr.unlink unless attr_name =~ DATA_ATTR_PATTERN
-                else
-                  attr.unlink
-                end
+            return
+          end
+
+          valid_attributes = Whitelist::ALLOWED_ATTRIBUTES[node.name]
+          return unless valid_attributes
+
+          node.attribute_nodes.each do |attr|
+            attr_name = attribute_name_with_namespace(attr)
+
+            if valid_attributes.include?(attr_name)
+              attr.unlink if unsafe_href?(attr)
+            else
+              # Arbitrary data attributes are allowed.
+              unless allows_data_attribute?(node) && data_attribute?(attr)
+                attr.unlink
               end
             end
           end
         end
+
+        def attribute_name_with_namespace(attr)
+          if attr.namespace
+            "#{attr.namespace.prefix}:#{attr.name}"
+          else
+            attr.name
+          end
+        end
+
+        def allows_data_attribute?(node)
+          Whitelist::ALLOWED_DATA_ATTRIBUTES_IN_ELEMENTS.include?(node.name)
+        end
+
+        def unsafe_href?(attr)
+          attribute_name_with_namespace(attr) == 'xlink:href' && !attr.value.start_with?('#')
+        end
+
+        def data_attribute?(attr)
+          attr.name.start_with?('data-') && attr.name =~ DATA_ATTR_PATTERN && attr.namespace.nil?
+        end
       end
     end
   end