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Cross-site Scripting occurrences in Random Banner
The Random Banner WordPress plugin is vulnerable to Stored Cross-Site Scripting due to insufficient escaping via the category parameter found in the ~/include/models/model.php file which allowed attackers with administrative user access to inject arbitrary web scripts, in versions up to and including 4.1.4. This affects multi-site installations where unfiltered_html is disabled for administrators, and sites where unfiltered_html is disabled. (2022-01-18, CVE-2022-0210)
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>