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Cross-site Scripting occurrences in Sign-up Sheets
The Sign-up Sheets WordPress plugin before 1.0.14 did not sanitise or escape some of its fields when creating a new sheet, allowing high privilege users to add JavaScript in them, leading to a Stored Cross-Site Scripting issue. The payloads will be triggered when viewing the 'All Sheets' page in the admin dashboard (2021-07-12, CVE-2021-24440)
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>