sanitize dynamic document attributes

app/javascript/template_builder/dynamic_editor.js

@@ -98,11 +98,29 @@ dynamic-variable {
   overflow-wrap: anywhere;
 }`)
 
+const DROP_ATTRS = [
+  'srcdoc', 'xlink:href', 'srcset', 'action', 'formaction', 'poster',
+  'background', 'data', 'cite', 'ping', 'longdesc', 'manifest', 'profile'
+]
+
+const SAFE_URL_REGEXP = /^(?:https?:\/\/|data:image\/|blob:|mailto:|tel:|\{)/i
+
+function isSafeAttr (name, value) {
+  const lowerName = name.toLowerCase()
+
+  if (lowerName.startsWith('on') || DROP_ATTRS.includes(lowerName)) return false
+  if ((lowerName === 'href' || lowerName === 'src') && !SAFE_URL_REGEXP.test(value.trim())) return false
+
+  return true
+}
+
 function collectDomAttrs (dom) {
   const attrs = {}
 
   for (let i = 0; i < dom.attributes.length; i++) {
-    attrs[dom.attributes[i].name] = dom.attributes[i].value
+    const { name, value } = dom.attributes[i]
+
+    if (isSafeAttr(name, value)) attrs[name] = value
   }
 
   return { htmlAttrs: attrs }