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Cross-site Scripting occurrences in Contact Form Check Tester
The Contact Form Check Tester WordPress plugin through 1.0.2 settings are visible to all registered users in the dashboard and are lacking any sanitisation. As a result, any registered user, such as subscriber, can leave an XSS payload in the plugin settings, which will be triggered by any user visiting them, and could allow for privilege escalation. The vendor decided to close the plugin. (2021-05-06, CVE-2021-24247)
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>