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Cross-site Scripting occurrences in Wordfence
** DISPUTED ** The Wordfence plugin 7.2.3 for WordPress allows XSS via a unique attack vector. NOTE: It has been asserted that this is not a valid vulnerability in the context of the Wordfence WordPress plugin as the firewall rules are not maintained as part of the Wordfence software but rather it is a set of rules hosted on vendor servers and pushed to the plugin with no versioning associated. Bypassing a WAF rule doesn't make a WordPress site vulnerable (speaking in terms of software vulnerabilities). (2019-04-25, CVE-2019-9669)
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>