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Cross-site Scripting occurrences in Wordpress Email Template Designer
The WP HTML Mail WordPress plugin is vulnerable to unauthorized access which allows unauthenticated attackers to retrieve and modify theme settings due to a missing capability check on the /themesettings REST-API endpoint found in the ~/includes/class-template-designer.php file, in versions up to and including 3.0.9. This makes it possible for attackers with no privileges to execute the endpoint and add malicious JavaScript to a vulnerable WordPress site. (2022-02-04, CVE-2022-0218)
Why Cross-site Scripting can be dangerous
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>