Upload Cross-site Scripting

FriendsofFlarum (FoF) Upload is an extension that handles file uploads intelligently for your forum. If FoF Upload prior to version 1.2.3 is configured to allow the uploading of SVG files ('image/svg+xml'), navigating directly to an SVG file URI could execute arbitrary Javascript code decided by an attacker. This Javascript code could include the execution of HTTP web requests to Flarum, or any other web service. This could allow data to be leaked by an authenticated Flarum user, or, possibly, for data to be modified maliciously. This issue has been patched with v1.2.3, which now sanitizes uploaded SVG files. As a workaround, remove the ability for users to upload SVG files through FoF Upload. (2022-06-02, CVE-2022-30999)

Cross site scripting is an attack where a web page executes code that is injected by an adversary. It usually appears, when users input is presented. This attack can be used to impersonate a user, take over control of the session, or even steal API keys.

The attack can be executed e.g. when you application injects the request parameter directly into the HTML code of the page returned to the user:

https://server.com/confirmation?message=Transaction+Complete

what results in:

<span>Confirmation: Transaction Complete</span>

In that case the message can be modified to become a valid Javascript code, e.g.:

https://server.com/confirmation?message=<script>dangerous javascript code here</script>

and it will be executed locally by the user's browser with full access to the user's personal application/browser data:

<span>Confirmation: <script>dangerous javascript code here</script></span>