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Cross-site Scripting occurrences in Redi Restaurant Reservation
The ReDi Restaurant Reservation WordPress plugin before 21.0426 provides the functionality to let users make restaurant reservations. These reservations are stored and can be listed on an 'Upcoming' page provided by the plugin. An unauthenticated user can fill in the form to make a restaurant reservation. The form to make a restaurant reservation field called 'Comment' does not use proper input validation and can be used to store XSS payloads. The XSS payloads will be executed when the plugin user goes to the 'Upcoming' page, which is an external website https://upcoming.reservationdiary.eu/ loaded in an iframe, and the stored reservation with XSS payload is loaded. (2021-05-17, CVE-2021-24299)
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>