Stop Spammers Cross-site Scripting

The Stop Spammers Security | Block Spam Users, Comments, Forms WordPress plugin before 2021.18 does not escape some of its settings, allowing high privilege users such as admin to set Cross-Site Scripting payloads in them even when the unfiltered_html capability is disallowed (2021-09-06, CVE-2021-24517)

The Stop Spammers WordPress plugin before 2021.9 did not escape user input when blocking requests (such as matching a spam word), outputting it in an attribute after sanitising it to remove HTML tags, which is not sufficient and lead to a reflected Cross-Site Scripting issue. (2021-05-06, CVE-2021-24245)

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>