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Cross-site Scripting occurrences in Yuzo
The Yuzo Related Posts plugin 5.12.94 for WordPress has XSS because it mistakenly expects that is_admin() verifies that the request comes from an admin user (it actually only verifies that the request is for an admin page). An unauthenticated attacker can inject a payload into the plugin settings, such as the yuzo_related_post_css_and_style setting. (2019-05-09, CVE-2019-11869)
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>