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Cross-site Scripting occurrences in Contact-form-7-datepicker
Stored XSS in the Contact Form 7 Datepicker plugin through 2.6.0 for WordPress allows authenticated attackers with minimal permissions to save arbitrary JavaScript to the plugin's settings via the unprotected wp_ajax_cf7dp_save_settings AJAX action and the ui_theme parameter. If an administrator creates or modifies a contact form, the JavaScript will be executed in their browser, which can then be used to create new administrative users or perform other actions using the administrator's session. (2020-04-07, CVE-2020-11516)
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>