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Cross-site Scripting occurrences in Motopress-slider-lite
The Responsive WordPress Slider WordPress plugin through 2.2.0 does not sanitise and escape some of the Slider options, allowing Cross-Site Scripting payloads to be set in them. Furthermore, as by default any authenticated user is allowed to create Sliders (https://wordpress.org/support/topic/slider-can-be-changed-from-any-user-even-subscriber/, such settings can be changed in the plugin's settings), this would allow user with a role as low as subscriber to perform Cross-Site Scripting attacks against logged in admins viewing the slider list and could lead to privilege escalation by creating a rogue admin account for example. (2021-10-25, CVE-2021-24544)
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>