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Cross-site Scripting occurrences in Stylish Cost Calculator
The Stylish Cost Calculator WordPress plugin before 7.0.4 does not have any authorisation and CSRF checks on some of its AJAX actions (available to authenticated users), which could allow any authenticated users, such as subscriber to call them, and perform Stored Cross-Site Scripting attacks against logged in admin, as well as frontend users due to the lack of sanitisation and escaping in some parameters (2021-11-29, CVE-2021-24822)
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>