Validate allowed schemes on preview card URLs (#27485)

app/models/preview_card.rb

@@ -55,7 +55,7 @@ class PreviewCard < ApplicationRecord
 
   has_attached_file :image, processors: [:thumbnail, :blurhash_transcoder], styles: ->(f) { image_styles(f) }, convert_options: { all: '-quality 90 +profile "!icc,*" +set date:modify +set date:create +set date:timestamp' }, validate_media_type: false
 
-  validates :url, presence: true, uniqueness: true
+  validates :url, presence: true, uniqueness: true, url: true
   validates_attachment_content_type :image, content_type: IMAGE_MIME_TYPES
   validates_attachment_size :image, less_than: LIMIT
   remotable_attachment :image, LIMIT

spec/models/preview_card_spec.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe PreviewCard do
+  describe 'validations' do
+    describe 'urls' do
+      it 'allows http schemes' do
+        record = described_class.new(url: 'http://example.host/path')
+
+        expect(record).to be_valid
+      end
+
+      it 'allows https schemes' do
+        record = described_class.new(url: 'https://example.host/path')
+
+        expect(record).to be_valid
+      end
+
+      it 'does not allow javascript: schemes' do
+        record = described_class.new(url: 'javascript:alert()')
+
+        expect(record).to_not be_valid
+        expect(record).to model_have_error_on_field(:url)
+      end
+    end
+  end
+end